Extending the dialect labeler (#2517) to replace hand-applied triage labels.

[LucaCappelletti94]

At this time, we do not have a structured PR template, and I have been applying/deleting manually the "waiting on contributor/reviewer" labels, and I am already falling behind. I think they are useful as a mechanism to easily see which PRs need reviewing and which do not.

I suggest to add the following mechanisms:

• (PR Template) Template lines: Reference:, Issues:, Fuzzed:, a kind checklist, reminders to search for duplicate PRs and issues, and an "I solemnly swear this is not slop" checkbox, with agent-written code being fine if adequately reviewed by its author. Anything maintainers deem slop gets closed.
• PR Labels: kind (the names generate-changelog.py reads, plus ci, fuzz harness, fuzz corpus), resolves issue/partial issue, and unfuzzed (a bug fix or new syntax without a new fuzz/fuzz_seeds/ entry).
• CI Bot-owned state:
  • waiting on reviewer when ready for review.
  • waiting on contributor plus conversion to draft on CHANGES_REQUESTED.
  • ready to merge on approval of the head commit.
  • needs rebasing on conflict.
• stale.yml ignores time spent waiting on reviewers, so we do not close PRs whose only fault is us being slow with reviews.

@alamb @iffyio opinions?