Migrate classic menus to block-based menus on theme switch

[adamziel]

Description

Adds an action that acts on theme_switch and:

• Short-circuits if the new theme isn't a FSE theme
• Finds all the menus assigned to a location in the old theme
• Migrates each to a wp_navigation post supported by the navigation block
• Updates fse_navigation_areas to reflect these migrated menus

gutenberg_migrate_nav_on_theme_switch is the only new function, the other ones were copied over from the navigation block. It would be great to remove the duplication in a follow-up PR.

How has this been tested?

• Switch to any classic theme
• Setup a few menus in a few locations, the locations should be called either primary, secondary, or tertiary
• Switch to tt1-blocks
• Confirm the menus were migrated as expected

[draganescu]

LGTM! 🚢

[getdave]

Works well. I noticed one thing. When you add a new Nav block you get duplicate choices under the Select existing menu dropdown in the placeholder state:

[adamziel]

Good find @getdave, let's address this in a follow-up PR

[draganescu]

Merged with "admin" because all checks were green except build artefact with was stuck on some network problem.

[spacedmonkey]

What about menus that used in widgets / hardcoded in templates. Locations are not the only way to know if a menu is being used.

[getdave]

Good find @getdave, let's address this in a follow-up PR

Did we create a follow up or should I look into that? I created a follow up in #36307

[getdave]

What about menus that used in widgets / hardcoded in templates. Locations are not the only way to know if a menu is being used.

Nonetheless, I'd say the locations route covers the 80% use case. Widgets/hard coding is possible, and completely valid, but seems less likely. Do you have any ideas for route we could all explore that could help cover those scenarios [the ones you mentioned] as well?