Design unified object for keyboard shortcuts and context menu actions

[rachel-fenichel]

As part of #130, we need to register all keyboard shortcuts (that should be accessible from context menus) as context menu items, with appropriate callbacks and preconditions.

Since the shortcuts are all defined in as an array of ShortcutRegistry.KeyboardShortcut items in navigation_controller.ts, it would be best to use that as the source of truth and write a helper to do the mapping, which is called once.

Later when we add a different scope to the context menu system we can update this helper to add the correct scope.