Ability to approve individual files

ok so the general idea is to allow approving of individual files inside a pull request, say I have 10 files and I approve 5 of them, I will not have to re-review them in case of new commits.

the catch - if a developer pushes a new commit that affects a file I already approved, I'd want the approval to be removed so I will re-review it.

but as a start I'd be happy to see the option to approve individual files, regardless if they might change later (since at the moment the same logic applies at the global level of the pull request)