Reusable blocks: Display outlines

[jameskoster]

In the Site Editor we've been using outlines to help illustrate the boundaries of a template part when it or its children have been selected.

Since template parts and reusable blocks are very similar, it might be worth creating UX alignment between these features. Especially since some of the changes in #27890 degraded the UX a little.

Here's how the outlines would work if we copied the current template part implementation.

Reusable block hovered

A 1px outline in the theme color appears on hover.

Reusable block selected and focussed

The hover outline is increased to 1.5px.

Reusable block selected, not focussed

A 1px outline in $gray-900 appears.

Reusable block child selected

A 1px outline in $gray-900 on the reusable block persists when child blocks are selected.

[jasmussen]

Good ticket, I did some initial work in #28283, and am happy to tweak to expand it.

Edit: I added the wrong link!

[mtias]

Marking this as done now that we have the overlay support.