feat: linear gradient px and transition hint syntax support (#48410)

Summary:
- Adds support for color transition hint syntax in linear gradients. e.g. `linear-gradient(red, 20%, green)`
- Adds `px` support. Combination of `px` and `%` also works.
- Simplified color stops parsing.
- The `processColorTransitionHint` and `getFixedColorStops` is moved to native code so it can support combination of `px` and `%` units as it requires gradient line length, which is derived from view dimensions and gradient line angle.
- Follows CSS [spec](https://drafts.csswg.org/css-images-4/#coloring-gradient-line) (Refer transition hint section) and implementation is referred from [blink engine source](https://github.com/chromium/chromium/blob/a296b1bad6dc1ed9d751b7528f7ca2134227b828/third_party/blink/renderer/core/css/css_gradient_value.cc#L240).

## Changelog:

[GENERAL] [ADDED] - Linear gradient color transition hint syntax and `px` unit support.

Pull Request resolved: #48410

Test Plan:
Added testcase in processBackgroundImage-test.ts and example in LinearGradientExample.js

<img width="500" alt="Screenshot 2025-01-05 at 11 38 13 PM" src="https://github.com/user-attachments/assets/62858bb7-1dbf-40cf-8dd4-ec0daf84ac1b" />

## Todo

Add testcases for `getFixedColorStops` and `processColorTransitionHint` in native code for both platforms. That's the only downside of moving it out of JS 🤦

Reviewed By: NickGerleman

Differential Revision: D67870375

Pulled By: joevilches

fbshipit-source-id: b91d741f3108c25df8000d220726bf180c64be60

packages/react-native/Libraries/StyleSheet/StyleSheetTypes.d.ts

@@ -372,7 +372,7 @@ export type GradientValue = {
   // Angle or direction enums
   direction?: string | undefined;
   colorStops: ReadonlyArray<{
-    color: ColorValue;
+    color: ColorValue | null;
     positions?: ReadonlyArray<string[]> | undefined;
   }>;
 };