Sort files in theme/plugin files tree

[pross]

Its a fairly trivial change to make the files alphabetically sortable fort both the theme and plugin file editors.
I know the file editor is never recommended to be used and is better off disabled but lets at least make it easier on the eyes?

This PR sorts the theme and plugin files alphabetically and in the case of the theme files it makes sure style.css functions.php and theme.json are always at the top of the tree.

Trac ticket: https://core.trac.wordpress.org/ticket/59616

[codente]

Love this, it would be so helpful!