ci: add workflow for PR automation

[IKatsuba]

Part of #142

• when a PR is opened, author should be assigned automatically
• when a PR changes a package (e.g. plugin-eslint), the corresponding label should be assigned
• when a PR is linked to an issue, issue status should change to "In review"
• when a PR is linked to an issue, all of the issue's labels should be added to the PR
• when a PR is not linked to an issue, it should be added to Project board with "In review" status
• when a PR title is created or changed, it should be checked with Commitlint
• when a PR cannot be merged because of conflicts, a "has conflicts" label should be added (and removed once conflicts are resolved)

Make three integrations. Another has to be done in another PR.

[matejchalk]

Nice work 🙂 Good idea to add the "testing" and "CI/CD" labels, too 👍

[Tlacenka]

I created an example PR and tried it out, nice QoL improvements! 🎉

The author self-assignment works. ✔️
The PR title commit lint also works well. ✔️
The only issue I see is the recognition of testing/production code/CI paths which were already mentioned by Matěj. 🎨

[BioPhoton]

Nice automations!