fix: ignore fork discard() after commit()

[Rich-Harris]

small tweak that fixes a gotcha I just encountered working with forks — if you call discard() on a fork that has already been committed (e.g. forks.forEach((f) => f.discard()), without removing the just-committed fork from forks) it will destroy any effects that are needed when the batch eventually resolves

no test because i am under a lot of time pressure 😬 but i consider a regression unlikely

also turned some lets and consts to vars, as is our wont. i also made double-commits no-ops, just like commit-discard.

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