proposal: spindle workflow cron trigger

Spindle workflows allow you to specify trigger events like push, pull_request, and manual: https://docs.tangled.org/spindles#trigger.

It would be really nice to be able to trigger workflows periodically, such as for automated dependency update checks or even as a convenient way to keep a tangled repository up to date with some remote.

Such an event trigger could look like this:

when:
  - event: ["schedule"]
    cron: "0 0 * * 0"

As a point of comparison, here's GitHub's docs for scheduled GitHub actions: https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#schedule