Add merge request CI for linting

There has been some talk of using gitlab CI to automatically lint test merge requests.

I wouldn't mind putting this together, using the existing tb-dev script to do some lint checking.

But there are a few things to figure out:

Gitlab runs the script in a container, so we would need a way to set up the environment so that ./mach lint would run. Gitlab also allows caching of files between runs, so we should figure out which files can be cached to speed up the set up.
What lints do we not want? "license" and "android-*", but anything else?
Sometimes the linter needs to pull in updates before it starts, but then fails to lint. How do we avoid this?
Which files should we check by default? I'm guessing only the files that have some change in the given merge request.
Any files that should always be excluded?
We want the linter to run for our own files, but what about when we modify a mozilla-central file?
We need to get the linter happy with our current code!
Do we want it to run automatically with each push, or only manually?
If you want to target the next ESR branch, you have to do two separate things: 1 rebase to the new branch and push, and 2 change the branch target in the merge request. There is a window of time between these two operations where gitlab thinks you are adding a very large number of commits, and these can happen in either order. So without any prevention, it would start lint checking a very large number of files.

Anyone have any suggestions? Or anything else we would need to figure out?