Display block shortcuts in Top Bar when Inserter is open

[jameskoster]

Extracted from #41379

When the Inserter is open, it could be nice to try replacing the document tools with shortcuts to insert common blocks. This will be most useful in situations where the Patterns tab is the default but you only want to insert a block.

It might be worth a little design exploration to work out details like animation, but that's probably a nice-to-have and needn't prohibit the creation of a PR.

[apeatling]

The issue I see with this -- if you select the wrong pattern and insert it there would be no obvious undo button. I can imagine this being a problem for people. It's easy in this interface to click the wrong pattern -- or see it on the canvas and change your mind without an obvious path out.

[apeatling]

I think having this, but also keeping the undo/redo buttons in place would be fine.

[jameskoster]

I think that could be okay, as long as there' some visual separation between the shortcuts and the undo/redo tools.