This just moves the code over to ci/github-script to make it easy to
test and iterate on locally.
The name `prepare` is chosen, because the script will be extended with
the other steps from "PR / prepare" next.
This turns the check-cherry-pick script into a github-script based
JavaScript program. This makes it much easier to extend to check reverts
or merge commits later on.
Since all github-scripts need to be written in commonjs, we now default
to it by not setting package.json. Support from editors for .js files is
slightly better than .cjs. To still allow using module imports in the
test runner script, we trick node into loading the script itself as a
module again via `--import ./run`.
