Nav Offcanvas inserter/appender button should provide more context

[getdave]

As reported in #46939 (comment) the accessible name Add block isn't a descriptive enough label for the Inserter without more context.

This should change to Add block to Navigation menu which would be preferable.

Also @talldan mentioned:

As Alex mention it would be good if the appender had some indication of context, blocks in List View have an aria-description like this - "Block 2 of 3, Level 1", which help give screenreader users an idea of order and nesting level, some extra information along these lines might be good. There might also be more that can be done, like announcing what the parent block is and that this appends to the end.