chore: make store initialization logic simpler

[Rich-Harris]

At the moment we have to explicitly check whether stores are initialized on every set, just to handle the edge case that we're setting up a store subscription. Much simpler to just to store.v = blah instead of set(store, blah) in that case, and it quarantines the complexity better

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[changeset-bot]

🦋 Changeset detected

Latest commit: 3f9b55b

The changes in this PR will be included in the next version bump.

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[dummdidumm]

should have a comment explaining why we need to do this

[Rich-Harris]

done