Experimental: Disable TinyMCE compat with classic editor

[tyxla]

What?

This PR makes the TinyMCE removal experiment compatible with the Classic Editor plugin and other plugins that disable the block editor.

Why?

In #50387 we introduced an experiment that allows us to remove TinyMCE from initial loading when we can. We're now polishing it in terms of plugin interoperability to make sure there are regressions and weird edge cases.

How?

We augment the existing gutenberg_post_being_edited_requires_classic_block() checks to also handle if the use_block_editor_for_post_type filter was used to disable the block editor specifically for a post type. This is the same filter that the Classic Editor plugin is using.

Testing Instructions

• Activate the "Disable TinyMCE" experiment if it's not active yet.
• Install and activate the "Classic Editor" plugin.
• Start writing a new post.
• Verify the TinyMCE editor loads and works correctly and there are no errors in the console.
• Disable the "Classic Editor" plugin.
• Start writing a new post.
• Verify you can see the block editor and there is still no "Classic" block available.

Testing Instructions for Keyboard

None

Screenshots or screencast

None

[dmsnell]

Looks like there's no harm in this. I'm a bit surprised that check is a function and not a filter, but that's not relevant to this change.

Thanks @tyxla for following up on this.

[tyxla]

I'm a bit surprised that check is a function and not a filter, but that's not relevant to this change.

Well, there's a filter with the same name so plugins can alter it, but there are a few more conditions under which the filter never runs. It's a bit odd because in those cases, the filter will not be considered. So the safer bet is to use the function:

https://github.com/WordPress/wordpress-develop/blob/f933aa3616aa8d49bdd67886e4af287579093c25/src/wp-includes/post.php#L8016-L8039

[github-actions]

Warning: Type of PR label error

To merge this PR, it requires exactly 1 label indicating the type of PR. Other labels are optional and not being checked here.

• Type-related labels to choose from: [Type] Accessibility (a11y), [Type] Automated Testing, [Type] Breaking Change, [Type] Bug, [Type] Build Tooling, [Type] Code Quality, [Type] Copy, [Type] Developer Documentation, [Type] Enhancement, [Type] Experimental, [Type] Feature, [Type] New API, [Type] Task, [Type] Performance, [Type] Project Management, [Type] Regression, [Type] Security, [Type] WP Core Ticket, Backport from WordPress Core.
• Labels found: [Type] Enhancement, [Type] Plugin Interoperability, [Type] Performance, [Block] Classic, [Type] Experimental.

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