Add action hook to Theme Editor

[biznis86]

Just before line 75 of file wp-admin/theme-editor.php, add action hook for developers to plug in. Perhaps sth like:
do_action('theme_editor_files');

This way, developers can hook into file editor and add their own files like stylesheets in subfolders and php files in level 2 subfolders.

Or just allow editing CSS files in subfolders by editing line 63 to
$style_files = $theme->get_files( 'css', 1 );

This way, level of security will stay the same and developers will gain more space to define theme file structure.

What's the point of having stylesheets in root theme folder anyway?

An action wouldn't be helpful there, a filter on $allowed_files would be.

Duplicate of #6531, #14175, and #17333.

Replying to SergeyBiryukov:

An action wouldn't be helpful there, a filter on $allowed_files would be.

Related: #22924