Add support for GitHub Actions style error output

[jemc]

GitHub Actions has a special style we can use when printing errors that will make the errors show up as annotations in the code of a pull request after they show up in a CI run.

This would be a relatively easy nice-to-have feature to make compiler errors (and in the future linter/warnings) more convenient in GitHub.

However, the format is not more readable for humans than our current format, so we'd want it to be an opt-in feature, or possibly one that we turn on if we detect (via env var perhaps) that we're running in a GitHub Actions environment.