Site Editor: return SlotFill for W button

[vindl]

Description

Initially we had a SlotFill for W button that allowed 3rd party apps to override this button in a maintainable way. Most notably this was useful for changing the back URL and site icon.

After the introduction, refactor, and removal of Browsing Sidebar this slot is no longer present, so the previous functionality has regressed.

This should've been included in #36194.

Here is how it previously looked

gutenberg/packages/edit-site/src/components/navigation-sidebar/navigation-panel/index.js

Lines 109 to 110 in 5c0b338

	<MainDashboardButton.Slot>
	<NavigationBackButton

.

Ideally we would use this same Slot/Fill here - similar to how the post editor uses its equivalent.

Step-by-step reproduction instructions

N/A

Screenshots, screen recording, code snippet

No response

Environment info

Gutenberg 11.9

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes