Add Github Actions to check for typos

[alexfikl]

This adds a very simple Github Actions setup that only runs typos. I mostly copied this over from some other project, so it has a few extra things:

• CI runs on pushes to main and PRs against main (the on: entries at the top).
• CI runs on every Sunday at 12PM (the cron job in there).
• in-progress CI jobs get cancelled if another commit was pushed to the same branch / PR (the concurrency: entry).

Also, since fypp isn't super active, the Github Actions CI thing will be automatically disabled after 3 months (I think) of inactivity and needs to be manually re-enabled after that. Just pushing a commit or a PR doesn't re-enable it, as far as I know.

xref: #59.