Add length (number) prop to whitelist AsyncGenerator and AsyncFunctionPrototype

[leotm]

Fix: #660
Fix: LavaMoat/docs#16

cc @naugtur

nb: will stick to 1 commit in future

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Before

After

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looks like checks CI / test-async-hooks failing on GHA setup-node matrix after adding docs (code comments)
so hopefully we only need to re-trigger the checks (i don't have permish from my fork)

Run actions/setup-node@v1
  with:
    node-version: 16.1
    always-auth: false
Premature close
Waiting 14 seconds before trying again
Premature close
Waiting 1[2](https://github.com/endojs/endo/actions/runs/4425617947/jobs/7760903288#step:3:2) seconds before trying again
Error: Premature close

edit: rebase + force-push worked ✅

[erights]

LGTM, thanks!