chore: simplify is_dirty

[Rich-Harris]

Another change extracted from #17362. I realised while reviewing it that there's no reason a reaction should ever be MAYBE_DIRTY if it has no dependencies. After a reaction with no dependencies first runs, it goes from DIRTY to CLEAN, and should never again be anything other than CLEAN.

The corollary is that we can simplify the code in is_dirty, because we know that reaction.deps is not null. Except that right now we do erroneously mark some effects MAYBE_DIRTY — legacy $: effects — so this PR fixes that too.

I'm not including a changeset with this one, because it's not making any user-observable changes (those MAYBE_DIRTY effects will never actually re-run, they will just unnecessarily go through the is_dirty process and be marked CLEAN) and falls under the #17362 umbrella. Self-merging once green to continue with that PR.

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