Run PRESUBMIT.py checks on GitHub PRs #64538

It would be great to require PRESUBMIT.py to run and pass before creating a Gerrit CL.

I guess we'd have to give it a security consideration, but I can't think of any cases where PRESUBMIT.py runs uploaded code. (I guess you'd have to skip PRESUBMIT.py if PRESUBMIT.py or any of its code dependencies is in the PR, lol.)

As it stands, someone uploads a PR, which is immediately converted to a Gerrit CL, and then nothing happens for a while. Then when a Dart engineer takes a look at the code and starts the trybots (because the uploader cannot do that themselves), you might see the most basic issues like code not being formatted.

Alternately, we could add some git-push hooks so that we check code formatting when a developer uploads a GitHub PR. These would hopefully mirror the PRESUBMIT.py checks as closely as possible.

CC @bwilkerson