WIP: Add filter to enable custom classes in "editor-styles-wrapper"

[colinduwe]

What?

Adds a 'bodyClasses' property to the block editor settings that emulates the classes applied to the pre-Gutenberg TinyMCE iframe and applies those classes to the block editor's iframe. This allows, for example, targeting CSS for the iframe based on the page template selected.

Why?

Addresses #17854 and #56831

How?

Adds a 'bodyClasses' property to the block editor settings that emulates the classes applied to the TinyMCE iframe.
The body classes can be modified by extenders via the block_editor_settings_all filter
The body classes are applied to the post/site editor iframe.
The body classes can be modified by updating the editor settings state. For example, when using the "swap template" or "use default template" buttons, the page template class is updated on the iframe.

Questions/Problems

1. This basically copies the code from TinyMCE to add the following classes: post-type, post-status, post-format, page-template, and locale. Are those still the classes we want added to the iframe?
2. Because attributes added to get_block_editor_settings are not available on the core/block-editor store, only on the deprecated core/editor store, how can this be modified to be merged?
3. This only modifies the page-template class when the user swaps the template or restores the default template. Creating a new template is still to do. Are there other actions that should update the body classes?
4. I couldn't figure out how to reference the 'core/editor' store by variable so I just kept referring to it by string and I use --no-verify to ignore the js lint errors.

[github-actions]

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[github-actions]

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Co-authored-by: colinduwe <[email protected]>

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