Add default option with default to true to clear out the vote on PR synchronize event

[myyk]

New votes are already triggered on

    types: [opened, synchronize, reopened] // these are defaults for pull_request_target if not specified explicitly

It would be better to make a few changes:

1. reopened perhaps should create a new vote since maybe it was closed for a while and old votes might not make sense anymore.
2. synchronize should somehow close other prior votes by marking them very explicitly as not an ongoing vote and create a new voting comment.
3. If not automatic in (2) we should make sure that the new vote comment is used throughout.