Consider updating the Code Editor design

[melchoyce]

Last night I took a stab at redesigning the Theme and Plugin code editors to match some of Gutenberg's new design patterns. I don't expect us to get around to this during 4.9, since I imagine it'll take a bit of work (and we probably shouldn't match Gutenberg's design before Gutenberg is out), but I thought it could be fun to bring an updated design to match the new syntax highlighting we'll be introducing.

Please note: these are first drafts and very exploratory. I'm sure I've forgotten things, and there's a lot that hasn't been designed yet. Consider this just a thought exercise for now. :)

PHP

CSS

[kenashworth]

Related: https://core.trac.wordpress.org/ticket/6531

(Would effect the nesting level.)

[westonruter]

Also related: https://core.trac.wordpress.org/ticket/24048 (Increase the usability of Plugin Editor's files list (separate directories, limit height))