feat(lua)!: register_keystroke_callback => on_key

Analogous to nodejs's `on('data', …)` interface, here on_key is the "add
listener" interface.

ref 3ccdbc570d #12536

BREAKING_CHANGE: vim.register_keystroke_callback() is now an error.

src/nvim/getchar.c

@@ -1563,8 +1563,8 @@ int vgetc(void)
    */
   may_garbage_collect = false;
 
-  // Exec lua callbacks for on_keystroke
-  nlua_execute_log_keystroke(c);
+  // Execute Lua on_key callbacks.
+  nlua_execute_on_key(c);
 
   return c;
 }