Add dictionary mapping command_id to its associated MenuCommands for each plugin

[DragaDoncila]

Adding this dictionary to support quick mapping of widgets/commands to their menus for implementing contributable menus . Prior to this PR, you would have to search all menu contributions for the command of the contribution you were currently processing. This map allows you direct access to the menus this command needs to live in.

This is a precursor to potentially rearchitecting the manifest schema entirely to be "command-first".

[jni]

Ah this is backwards. You should be able to do this in a single pass of the menus:

• iterate over the menus, but
• map commands back to menus
• since you use a defaultdict it's ok if some menu-less commands are missed, but you could add them by keeping a set of already-seen commands and then putting in the missed ones at the end.

So:

Suggested change

	for command in manifest.contributions.commands or ():
	# command IDs are keys in map
	# each value is a dict menu_id: list of MenuCommands
	# for the command and menu
	associated = self._get_associated_menus(manifest, command.id)
	self._command_menu_map[manifest.name][command.id] = associated
	def _get_associated_menus(
	self, manifest: PluginManifest, command_id: str
	) -> Dict[str, List[MenuCommand]]:
	menus = manifest.contributions.menus or dict()
	associated_menus = defaultdict(list)
	for menu_id, items in menus.items():
	for item in items:
	if getattr(item, "command", "") == command_id:
	associated_menus[menu_id].append(item)
	return associated_menus
	menu_map = self._command_menu_map[manifest.name] # just for conciseness below
	for menu_id, menu_items in manifest.contributions.menus.items() or ():
	# command IDs are keys in map
	# each value is a dict menu_id: list of MenuCommands
	# for the command and menu
	for item in menu_items:
	if command_id := getattr(item, "command", None) is not None:
	menu_map[command_id].append(item)