Ensure Hello Dolly plugin text doesn't overlap the widgets screen UI

[getsource]

Description

Consolidates both classes necessary (block-editor-page and wp-embed-responsive) into one function to add required admin body classes for the new Widgets editor, gutenberg_widgets_editor_add_admin_body_classes().

Only calls add_filter() when widgets editor page is being loaded.

Closes: #26219

How has this been tested?

Tested manually with Hello Dolly in wp-env, along with existing automated tests with npm run test.

Screenshots

One of the issues this fixes is Hello Dolly text being overlapped by the editor:
Before:

After:

Types of changes

Primarily a bug fix to resolve the Hello Dolly issue above, but includes behavior changes.
block-editor-page is added to the new Widgets screen, which would mean styles targeted to that class will now be applied.

Changes the way the wp-embed-responsive class is added, as it is now only added when the widget page is loaded, instead of adding it to every admin page. I believe this is what was intended, based on the previous function's wording.

As part of this, an existing function, gutenberg_widgets_editor_add_responsive_embed_body_class is removed in favor of gutenberg_widgets_editor_add_admin_body_classes to add both classes.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[talldan]

Thanks Mike, this does the trick 🎉

[talldan]

Just renaming the PR, as the title ends up in the plugin changelog, so it's good to have something more user focused.

[getsource]

That's good to know -- thank you!