"File Finder" improvements #2467

1. Visit https://github.com/sindresorhus/refined-github
2. Press t
3. A file finder will load

There are 3 improvements that can be made:

• t only works while you're on /tree/ and /blob/ pages; it should work on every page of the repo, including issues.
• When navigating back to the File Finder (e.g. open finder, open file from list, go back to finder), the previous input is lost; this should instead be preserved.
• After pressing t, you have to wait for the page to load before typing a file name; usually software will buffer your keyboard input even if the UI takes a while to load; we should simulate the same behavior so you can type t and start typing your query right away.