Lacking an appropriate action in the classic editor, plugins that add meta boxes have historically hooked into various actions in order to add hidden input fields.
This change also adds backwards compatibility for two of the most common: `edit_form_after_title`, and `edit_form_advanced`.
Props pento, danielbachhuber.
See #45283.
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The block editor needs to duplicate the classic meta box behaviour, so it can extract the registered meta boxes, and import them into the block editor.
To match the classic editor behaviour as closely as possible, this moves the relevant code from the classic editor, into a new function, so it can be called by both.
Props pento, peterwilsoncc.
Fixes#45172.
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