watchers: Add a new event on_color_scheme_preference_change

Fixes #8246

docs/launch.rst

@@ -168,6 +168,13 @@ create :file:`~/.config/kitty/mywatcher.py` and use :option:`launch --watcher` =
         # data will contain is_start, cmdline and time.
         ...
 
+     def on_color_scheme_preference_change(boss: Boss, window: Window, data: dict[str, Any]) -> None:
+        # called when the color scheme preference of this window changes from
+        # light to dark or vice versa. data contains is_dark and via_escape_code
+        # the latter will be true if the color scheme was changed via escape
+        # code received from the program running in the window
+        ...
+
 Every callback is passed a reference to the global ``Boss`` object as well as
 the ``Window`` object the action is occurring on. The ``data`` object is a dict
 that contains event dependent data. You have full access to kitty internals in