Add feature to approve code by file

Sometimes, a PR is massive, or has updates to multiple files. It would be nice to have the ability to approve files as you go through them. For instance, if a PR has 5-35 files in it, as you review a file, there could be an "approve" button at the top of the file (like where the "side-by-side", "view file", and "comment" buttons are.
Ideally, the user would review this code, find no error, click approve...and this file field would collapse to one line.
This would make the code review process easier and help shorten the screen for long pull requests.