`use_block_editor_for_post_type` and `use_block_editor_for_post` can be very useful in more contexts than wp-admin, especially when a site is in transition. For example, you may want to do things on init that are different.
Neither function depends on other functions that are available only in wp-admin (other than use_block_editor_for_post() relying on use_block_editor_for_post_type() and an admin-referrer check that's historically gated by a query variable and now also gated by is_admin), therefore moving them to wp-includes seems both feasible and beneficial
Props ethitter, jorbin.
Fixes#51819.
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