Create test workflow using web.dev and d.c.c repos

[matthiasrohmer]

With the effort started in #36 I think it would be good to run builds for both web.dev and d.c.c for changes like this in a workflow running in this repo.

This has the advantage that we can quickly and easily test perf impacts here and take accurate measurements, without needing to publish beta versions and create test PRs on the actual site repos.