Weekly "canary" builds via GitHub Actions

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#onschedule

I'd really like to set up a weekly build against release candidates of [TypeScript](https://www.npmjs.com/package/typescript?activeTab=versions) and ts-morph. The idea is when I'm away, we can catch bustages up front.

This is more of a "nice to have" than an actual requirement.