feat: 🎸 add GitHub welcome action to welcome new contributos

[trueberryless]

Description

Add a new welcome bot GitHub action which runs on new merge requests and checks if this is the first interaction of the user with the repository. If so, it will write a lovely welcome message, assuring that the user did everything correctly and that a review can take some time. 🙌

Why?

I think this action makes new contributors feel welcomed!

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[JimMadge]

A few questions about the actions, but this looks good to me. I like the idea of having something to welcome and inform new contributors 👍

[trueberryless]

Actually, I just found out that the actions/first-interaction requires the issue_message parameter alto it shouldn't be (see this issue), so reverted the last commit for now.

[JimMadge]

Let's remove the checkout step if it isn't needed.

Shame that the upstream project has a bug. Can we use an older version? Or maybe wait for actions/first-interaction#404 to be merged?

[trueberryless]

Shame that the upstream project has a bug. Can we use an older version? Or maybe wait for actions/first-interaction#404 to be merged?

However you'd like. I kinda doubt that my PR in the upstream action will be merged any time soon, so I wouldn't bet on waiting for it. And I can also confirm that the fork action works, as we have it running in Starlight and also npmx. But I'll leave the decision up to you whether you wanna use the fork in the meantime or rather have no welcome until the official action works 👍