What problem does this address?

This issue is a follow-up to #34612. Currently, that issue has two PRs exploring solutions, one using template part post types (#35418) and another using a separate wp_navigation post type (#35746).

When switching themes, it'd be good to try to preserve navigation block menu data. The previously mentioned issue explores saving navigation block data to a post. This would make it possible to retain that data when a user switches themes.

There are some different permutations
a) migrating from a classic menu (associated with a location) to a navigation block in a block-based theme (associated with a template part)
b) migrating between block-based themes

One of the challenges is that in a block-based theme the Nav Block is just rendered in a template part as HTML. It doesn't have the concept of a 'location'. This might be something we need to implement.