Add 'Open CSS' command to the command centre

[dsas]

What?

Add the 'Open CSS' command to the command centre when in the site-editor

Why?

One of the command centre ideas mentioned in #51502

How?

I've branched from @youknowriad's #51637

Testing Instructions

1. Open the site editor
2. Open the command centre, e.g. with ⌘ + k
3. Type 'CSS'
4. See the command appear in the list
5. Choose the command
6. Watch the 'Additional CSS' screen open.

Post editor

Try it again in the post editor and see that it doesn't appear and you can't use it :)

Without edit_css capabilities

Now try it again but as a user that does have access to the site editor, but doesn't have access to custom CSS.

Add a snippet like this:

add_filter( 'map_meta_cap', 'no_css', 10, 2 );

function no_css( $caps, $cap ) {

	if ( 'edit_css' === $cap ) {
		$caps = array( 'do_not_allow' );
	}

	return $caps;
}

Now try try to run the steps again, you should notice that Open CSS doesn't show up in the command list, just as it doesn't show up in the GS sidebar & three-dot menu

Testing Instructions for Keyboard

As above

Screenshots or screencast

Screen.Capture.on.2023-06-20.at.22-58-24.mp4

[youknowriad]

This is the first time I see this effect. It's a bit weird that this relies on goTo.

I had an idea that I needed elsewhere too to solve this more elegantly: Allow NavigatorProvider to work as a controlled component. I think that's something we'll be needing anyway in the site editor to avoid race conditions when synchronizing the URL to the state. ( cc @ciampo )

(This is not for this PR)

[youknowriad]

This is looking good to me

[youknowriad]

Going to merge it to my PR so we can merge both together.