fix bind:group

[iacore]

Hopefully fixes #2308

repo i used to test this fix: https://github.com/iacore/fix-svelte-bind-group

the code using new Set() feels inefficient though. it might be better to make value in bind:group=value a Set.

[changeset-bot]

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Latest commit: 26e9636

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[Rich-Harris]

It looks like this breaks a bunch of tests. It's not totally clear what's going on here — if you're still interested in working on this, mind adding a test that fails without this change?

Marking as draft in the meantime