ci: preview release via pkg.pr.new for current packages

[ematipico]

Changes

This PR revamps the pkg.pr.new that we had disabled.

Changes:

• triggers the job only on labeled event
• uses changeset status to retrieve the upcoming releases
• uses a script to parse the JSON emitted by the previous step, and return a list of packages that have changesets. It returns a string
• if the previous string is empty, we write a message in the PR saying that no preview release was published
• if the previous string isn't empty, it's passed to the pkg pr new CLI

Testing

I tested the script locally.
This PR needs to be merged, I will enable the job again, and then open a PR to test it.

Docs

N/A

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4dc1024

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

❌ Snapshot release failed.

[ascorbic]

You could probably do this without the script using jq on the command line, but it's not a big deal. (I always ask an LLM for the actual arguments because I can never remember the syntax)

[ematipico]

I evaluated jq, but for a project like Astro, having a JS script might be better for contributions and bug fixes