Create automatic binding to sign in forms

[Firehed]

I suspect a common pattern for sign in will look roughly like this:

<form ... onSubmit={SACallback}>
  <input type="text" autocomplete="username webauthn" />
</form>

const SACallback = (e) => {
  e.preventDefault()
  const username = document.querySelector('input[something]').value
  const auth = await sdk.startAuth({ handle: username })
  // ...
}

It may be possible to make a convenience API that looks more like handleAutofill and handles all of the event binding for the client.