Add a pre-publish check for DevTools extension validation

[kenzieschmoll]

This is a feature originally requested and discussed here: rrousselGit/provider#832 (comment).

This check would fail on pub publish --dry-run if a DevTools extension is expected and none was found.

[rrousselGit]

It'd be cool if the check could not only very that the build folder exists, but also check that the build output is more recent than the source files.

When thinking about this problem, I realized that gitignored files would still be present on the computer at the publish step. So if publishing locally instead of in a CI, the check could incorrectly pass.

[sigurdm]

When thinking about this problem, I realized that gitignored files would still be present on the computer at the publish step. So if publishing locally instead of in a CI, the check could incorrectly pass.

I think we can get around this problem by only considering files not gitignored.