Use GitHub Actions for checks

[StefMa]

Hello everyone ✌️

this PR might fix #152 and therefore also contributes to #225.

Two reasons why this would be a step forward:

• Forks can simply check in their own repo if something isn't broken. Right now they have to check stuff locally and we all know this might not happen all the time 😅
• PRs, depending on the setting, can run automatically run checks without an approval.
• Since this is the "start" of using GitHub actions, we can also start implemeting Cosinder adding dependabot for automatic dependency updates  #225 as dependabot would also run on actions and we can create workflows that "do something after they run" or after "dependabot created a PR"... See https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions
• ???

I just converted the checks from the circleci config for now because

1. I wanted to hear your opinion about this first (generally)
2. What do you think about using my pkl-gha project 🙃
3. Other comments 🙃
   Le me know if this is "fine" or if I also convert all the other jobs.
   On the other side we can also do this as follow-ups in other PRs.
   Let me know what you prefer.

Because using a matrix is super easy to setup on actions, I also decided to run jdk 21 on windows 🤷
This isn't in the circleci config but doesn't hurt i guess?!

If you want to see how this looks like in action, checkout this run on my fork:

• Check Pkl Actions Converted
• Checks

TODO before merge:

• Remove demosntrate github action run

[bioball]

Hi @StefMa, thanks for the PR!

We're considering switching to GitHub Actions, but we're not quite ready yet to accept a commit for this. However, you can always maintain this commit on your own fork for now as a way to build and test Pkl yourself.

However, this is quite useful for us (I've never used GHA before), so, let's keep this PR around so we can go from here when we're ready!

Also, I'm open to using pkl-gha :)

[madrob]

@bioball Actions are currently disabled on the repo, but would it be useful to merge anyway so that other folks who are forking can run the tests? I wouldn't want this to turn into a maintenance burden for the team to have untested code in the repo, but it could also be a nice quality of life improvement for contributors.