Handle non-direct insertion in Nav offcanvas inserter

[getdave]

In #46013 we added an inserter to the Nav offcanvas sidebar.

This works reasonably well when core/navigation-link blocks are auto-inserted which is the default behaviour of the Nav block (see this research).

However if you do the following you will get a different experience:

• use offcanvas inserter
• don't type a link into the Link UI
• click on one of the "Transforms" shown at the bottom of the Link UI
• see block inserted
• click inserter in offcanvas again
• see a full inserter with all the blocks that can be inserted into a Nav block

Again this is due to the behaviour I have previously detailed.

I'd like to know how we'd prefer to handle this situation in the offcanvas. Some design input would really help. Maybe @SaxonF can help here?

[getdave]

I will take this one as I've done the ground work already.