launch watcher: add on_cmd_startstop event

docs/launch.rst

@@ -145,6 +145,10 @@ functions for the events you are interested in, for example:
         # when a title change was requested via escape code from the program
         # running in the terminal
 
+    def on_cmd_startstop(boss: Boss, window: Window, data: Dict[str, Any]) -> None:
+        # called when the shell starts/stops executing a command. Here
+        # data will contain is_start and time.
+
 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. Some useful methods and attributes for the