Add filter to theme editor accepted files

[pross]

Proposing adding a filter to theme-editor.php so theme authors can add to the list of files that are editable. ie html/less/js files.

Hers an example of what could be possible:

add_filter( 'wp_theme_editor_filetypes', 'add_my_filetype', 10, 2 );
function add_my_filetype( $allowed_types, $theme ) {
	
	$extra = $theme->get_files( 'less', 1 );
	return $allowed_types += $extra;
}

Created a test.less file in my theme, added the patch and example. It worked great, I think this would be a good addition.

Related: #6531