Apply purple colour and "click through" to Nav block

[getdave]

In #46412 @jasmussen said:

Apply the purple "synced" color to the Nav block (both for any clickthrough overlays applied, but also to the icon and select state in the inspector).

We need to

• Wait for Add edit button to template parts, reusable blocks and navigation blocks #45465 to land
• Apply the purple outline coloring to the block (but not all the coloring in the List View...etc)
• Apply the "click through" behaviour to the block.

[talldan]

There's a PR from @youknowriad that does something like this, or something that conflicts with this - #45465.

It looks like the idea is to add a new support property __experimentalBlockOverlay, but it'll completely lock the inner blocks until an 'edit' button is clicked.