Navigation in Site View: Improve fallback loading

[scruffian]

What problem does this address?

When a site has no navigations, we load the fallback when you open the Navigation section of the Site View:

What is your proposed solution?

We should find a way to load the fallback earlier so that its already where when you navigate to the the Navigation section.

To replicate:

1. Delete all your navigations
2. Open the site editor
3. Open the navigation section
4. Notice that there is a small flash before the fallback is created.

[getdave]

May be fixed by #51795

It won't be "fixed" but we can extend it by using the special button as a place to preload the fallback if there are no menus.

We can even do it on mouseenter like this:

// ...snip
const { getNavigationFallbackId } = useSelect( coreStore );
// ...snip
<NavigatorButton
	as={ SidebarNavigationItem }
	path="/navigation"
	withChevron
	icon={ navigation }
	onMouseEnter={ () => {
		getNavigationFallbackId();
	} }
>

[getdave]

Or we could check on the resolution status of getNavigationFallbackId and if it hasn't resolved then show a spinner.

So that's change the condition for isLoading to also consider status of Navigation Fallback resolver.

[getdave]

PR to fix is open #52322