Theme Editor: Break `$wp_file_descriptions` array into sections and reorder...

Theme Editor: Break `$wp_file_descriptions` array into sections and reorder for consistency and readability.

Props ramiy.
Fixes #35223.
Built from https://develop.svn.wordpress.org/trunk@36088


git-svn-id: https://core.svn.wordpress.org/trunk@36053 1a063a9b-81f0-0310-95a4-ce76da25c4cd