feature request: remove LGTM approvals on new commit

[refack]

As per https://github.com/nodejs/node/pull/13413/files#r120732145 we could enable people to conditionally approve by using the LGTM comment (instead on GitHub's Approval).
That is:

• People who use GitHub Approval defer to the lander.
• People who opt to comment LGTM want to be dismissed if a new commit is added after their comment.

_{I'll open a PR myself, I just wanted to record this feature request}

[legendecas]

GitHub now has an option to dismiss all approvals on new commits. Though it might be too aggressive to apply the rule on all reviews.