Add a GitHub PR check for transformers/

Ensure that no PR includes a change to `transformers/` unless it is a sync from Google (e.g. #309).

(Ideally, we'll make GitHub the source of truth for `transformers/` eventually, but that's a harder project.)

Start by locating the repository's GitHub pull-request check configuration and reviewing how the sync from Google is identified. Define the check so changes under transformers/ are rejected unless they match that sync case; done means an ordinary change is blocked and the permitted sync passes.

Written by the indexing model from the issue text.