Add public hook eglot-managed-mode-hook

Per #354, fix #182.

* eglot.el (eglot-managed-p): New function.
(eglot--managed-mode-hook): Obsolete it.
(eglot-managed-mode-hook): New hook variable.
(eglot--managed-mode): Run the new hook.

* README.md (Customization): Mention the new hook.

README.md

@@ -290,6 +290,9 @@ lisp:
 - `eglot-server-initialized-hook`: Hook run after server is
   successfully initialized;
 
+- `eglot-managed-mode-hook`: Hook run after Eglot starts/stops
+  managing a buffer.  See its documentation for usage tips;
+
 # How does Eglot work?
 
 `M-x eglot` starts a server via a shell-command guessed from

eglot.el

@@ -1234,6 +1234,19 @@ For example, to keep your Company customization use
 (defvar-local eglot--cached-server nil
   "A cached reference to the current EGLOT server.")
 
+(defun eglot-managed-p ()
+  "Tell if current buffer is managed by EGLOT."
+  eglot--managed-mode)
+
+(make-obsolete-variable
+ 'eglot--managed-mode-hook 'eglot-managed-mode-hook "1.6")
+
+(defvar eglot-managed-mode-hook nil
+  "A hook run by EGLOT after it starts/stops managing a buffer.
+Use `eglot-managed-p' in the hook function to determine whether
+EGLOT starts or stops managing a buffer, it returns t or nil,
+respectively.")
+
 (define-minor-mode eglot--managed-mode
   "Mode for source buffers managed by some EGLOT project."
   nil nil eglot-mode-map
@@ -1289,7 +1302,9 @@ For example, to keep your Company customization use
               (delq (current-buffer) (eglot--managed-buffers server)))
         (when (and eglot-autoshutdown
                    (null (eglot--managed-buffers server)))
-          (eglot-shutdown server)))))))
+          (eglot-shutdown server))))))
+  ;; Note: the public hook runs before the internal eglot--managed-mode-hook.
+  (run-hooks 'eglot-managed-mode-hook))
 
 (defun eglot--managed-mode-off ()
   "Turn off `eglot--managed-mode' unconditionally."