rich-text: Fix bug where bare tag name format types could not be registered

[davilera]

What?

This PR fixes #46795. It also tweaks the test suite to account for the reported bug.

Why?

Users should be able to register format types to handle bare tag names. Unfortunately, this is currently not possible as described in #46795.

How?

When registering a new format type name/format, the code now checks if the existing format type that handles a bare tag name xxx is core/unknown. If it is, it means nobody registered a specific format type for tag name xxx and, therefore, name/format should be allowed.

Testing Instructions

1. Go to the post editor
2. Open your browser's developer tools
3. In the JavaScript console, type the following:

wp.richText.registerFormatType( 'name/format', {
  title: 'Test bare tag format',
  tagName: 'xxx',
  className: null,
  edit: () => null,
} );

It should work.

4. Then type the following:

wp.richText.registerFormatType( 'name/another-format', {
  title: 'Test bare tag format',
  tagName: 'xxx',
  className: null,
  edit: () => null,
} );

It should fail because tag name xxx is already handled by name/format (registered in step 3).

[noahtallen]

This change makes a lot of sense, and works great testing locally.

I tried the following code in the browser console:

wp.richText.unregisterFormatType( 'core/link' )

wp.richText.registerFormatType( 'my/link', {
  title: 'Test link format',
  tagName: 'a',
  className: null,
  edit: () => null,
} );

this fails on trunk, but works in this branch. Let's ship so that we can create a patch release! (I can handle that 👍 )

I think it makes sense to move forward on this quickly. If further changes are needed, I think those could be addressed in a follow-up, given that this is an important bug fix for plugin compatibility.