feat: iOS 12 compatibility (#764)

## 📜 Description

Make `useFocusedInputHandler` compatible with iOS < 13.

## 💡 Motivation and Context

The library was always compatible with really old iOS versions. However
there was a one handler (`onSelectionChange`) which was a stub on iOS <
13.

In this PR I'm adding support for old iOS versions for this handler via
`KVO` approach + dispatching events on text changes.

The new algorithm can be breaking into next steps:
- on saving a new reference to delegate we remove old KVO;
- on saving a new reference to delegate we add KVO for UITextField (iOS
< 13 only);
- we dispatch selection event from KVO/change text events (to be fully
backward comaptible with iOS 13+, because single KVO don't emit events
for text changes - only selection);
- when text editing has been completed - we remove KVO.

One thing I potentially don't really like a lot is that we remove KVO in
`endEditing` event. Starting from
#760
we will return delegate to its original variant on `endEditing` event,
but it seems like we'll have kind of transitive dependencies (the fact
that we receive this event in two places). However at the moment I can
not imagine a situation when we will not substitute delegate back when
we finish text editing, so theoretically this solution should be okay.

Closes
#763

## 📢 Changelog

<!-- High level overview of important changes -->
<!-- For example: fixed status bar manipulation; added new types
declarations; -->
<!-- If your changes don't affect one of platform/language below - then
remove this platform/language -->

### iOS

- `setTextFieldDelegate` accepts second `textField` param; 
- store `observedTextFieldForSelection` in composite delegate class;
- added KVO helpers function (to assure we can remove it only one time);
- remove KVO when new delegate is set;
- add new KVO when new delegate is set;
- send selection events from text change/kvo;

## 🤔 How Has This Been Tested?

Tested on macOS 12 with XCode 14 and iPhone X iOS 12.4 simulator.

## 📸 Screenshots (if appropriate):


https://github.com/user-attachments/assets/98d01bae-0d58-42e7-9658-ff382b692087

## 📝 Checklist

- [x] CI successfully passed
- [x] I added new mocks and corresponding unit-tests if library API was
changed

ios/delegates/KCTextInputCompositeDelegate.swift

@@ -51,6 +51,9 @@ class KCTextInputCompositeDelegate: NSObject, UITextViewDelegate, UITextFieldDel
   weak var textViewDelegate: UITextViewDelegate?
   weak var textFieldDelegate: UITextFieldDelegate?
 
+  // Keep track of which textField we’re observing (iOS < 13 only)
+  private weak var observedTextFieldForSelection: UITextField?
+
   public init(
     onSelectionChange: @escaping (_ event: NSDictionary) -> Void,
     onTextChange: @escaping (_ text: String?) -> Void
@@ -62,13 +65,35 @@ class KCTextInputCompositeDelegate: NSObject, UITextViewDelegate, UITextFieldDel
   // MARK: setters/getters
 
   public func setTextViewDelegate(delegate: UITextViewDelegate?) {
+    // remove KVO from any old textField
+    if let oldTextField = observedTextFieldForSelection {
+      removeSelectionRangeObserver(from: oldTextField)
+    }
+
     textViewDelegate = delegate
     textFieldDelegate = nil
+    observedTextFieldForSelection = nil
   }
 
-  public func setTextFieldDelegate(delegate: UITextFieldDelegate?) {
+  public func setTextFieldDelegate(delegate: UITextFieldDelegate?, textField: UITextField?) {
+    // remove KVO from any old textField
+    if let oldTextField = observedTextFieldForSelection {
+      removeSelectionRangeObserver(from: oldTextField)
+    }
+
     textFieldDelegate = delegate
     textViewDelegate = nil
+
+    // If iOS < 13, add KVO to the actual textField object
+    if #available(iOS 13.0, *) {
+      // rely on textFieldDidChangeSelection
+      observedTextFieldForSelection = nil
+    } else {
+      if let realTextField = textField {
+        addSelectionRangeObserver(to: realTextField)
+        observedTextFieldForSelection = realTextField
+      }
+    }
   }
 
   // Getter for the active delegate
@@ -116,9 +141,56 @@ class KCTextInputCompositeDelegate: NSObject, UITextViewDelegate, UITextFieldDel
       self.onTextChange(textField.text)
     }
 
+    if #unavailable(iOS 13.0) {
+      DispatchQueue.main.asyncAfter(deadline: .now() + UIUtils.nextFrame) {
+        updateSelectionPosition(textInput: textField, sendEvent: self.onSelectionChange)
+      }
+    }
+
     return textFieldDelegate?.textField?(textField, shouldChangeCharactersIn: range, replacementString: string) ?? true
   }
 
+  func textFieldDidEndEditing(_ textField: UITextField) {
+    textFieldDelegate?.textFieldDidEndEditing?(textField)
+
+    if #unavailable(iOS 13.0) {
+      removeSelectionRangeObserver(from: textField)
+      if observedTextFieldForSelection === textField {
+        observedTextFieldForSelection = nil
+      }
+    }
+  }
+
+  // MARK: KVO for iOS < 13
+
+  private func addSelectionRangeObserver(to textField: UITextField) {
+    textField.addObserver(
+      self,
+      forKeyPath: "selectedTextRange",
+      options: [.new],
+      context: nil
+    )
+  }
+
+  private func removeSelectionRangeObserver(from textField: UITextField) {
+    textField.removeObserver(self, forKeyPath: "selectedTextRange")
+  }
+
+  // swiftlint:disable:next block_based_kvo
+  override func observeValue(
+    forKeyPath keyPath: String?,
+    of object: Any?,
+    change: [NSKeyValueChangeKey: Any]?,
+    context: UnsafeMutableRawPointer?
+  ) {
+    guard keyPath == "selectedTextRange", let textField = object as? UITextField else {
+      super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
+      return
+    }
+    // selection changed => forward the event
+    updateSelectionPosition(textInput: textField, sendEvent: onSelectionChange)
+  }
+
   // MARK: call forwarding
 
   override func responds(to aSelector: Selector!) -> Bool {

ios/observers/FocusedInputObserver.swift

@@ -188,7 +188,7 @@ public class FocusedInputObserver: NSObject {
   private func substituteDelegate(_ input: UIResponder?) {
     if let textField = input as? UITextField {
       if !(textField.delegate is KCTextInputCompositeDelegate) {
-        delegate.setTextFieldDelegate(delegate: textField.delegate)
+        delegate.setTextFieldDelegate(delegate: textField.delegate, textField: textField)
         textField.delegate = delegate
       }
     } else if let textView = input as? UITextView {