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.

Built from https://develop.svn.wordpress.org/trunk@42112


git-svn-id: https://core.svn.wordpress.org/trunk@41941 1a063a9b-81f0-0310-95a4-ce76da25c4cd