Theme editor: Unable to edit files more than 1-level deep

[westonruter]

In [41806] for #6531 the logic for gathering the list of $allowed_files was modified to allow for a unlimited depth for listing out files to edit. However, the same change was not then likewise applied to wp_edit_theme_plugin_file() when actually validating the file path to edit. The result is that a user can view the code any file in a theme but they cannot save any edits they make. The logic in wp_edit_theme_plugin_file() needs to be updated to match the code in theme-editor.php.

[westonruter]

Aligns the logic between:
​https://github.com/WordPress/wordpress-develop/blob/f0cb7b5/src/wp-admin/theme-editor.php#L75-L90
and
​https://github.com/WordPress/wordpress-develop/blob/f0cb7b5/src/wp-admin/includes/file.php#L432-L446

[obenland]

42425.0.diff​ fixes the bug, LGTM.

[westonruter]

In 42112:

Theme Editor: Ensure files listed recursively can be both viewed and edited.

Prevent edits to 2-level deep theme files from returning a disallowed_theme_file error when attempting to save an edit. Aligns logic for gathering $allowed_files in theme-editor.php for listing files with the validation logic in wp_edit_theme_plugin_file().

Amends [41806].
See #6531.
Fixes #42425.