fix: handle deletions of state proxy properties

[Rich-Harris]

closes #13003. We were doing a couple of things wrong — the deleteProperty trap should always return true, not just when the property previously existed (I was surprised by this, but try it — delete {}.foo), and we need to create an UNINITIALIZED source when deleting a property that exists on the target otherwise it will show up in ownKeys

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[bartpype]

Thank you Rich for quick fix!