Setup initial end-to-end tests, end-to-end testing infrastructure

[tmillr]

Why?

• to make sure that action.yml is still valid on and into the future (e.g. its schema may change, the chosen/selected node version may become outdated/invalid, etc.)
• to make sure the action does not break when the gh runners (or their interface with js actions) change
• for end-to-end testing, if wanted or needed
• provides a way to capture/view the action's actual output from an actual/realistic run (e.g. for docs: include screenshot and link to example of posted comments #1)

How?

• create a scratch branch or scratch repo for pushing test commits to
  • if branch, the ci yaml itself must still be on the default branch
• choose any events necessary, but also at least run on a schedule (e.g. daily) since it's possible for this action (or its manifest: action.yml) to break even if the pkg/repo itself doesn't actually change
• observe workflow runs for failures, esp those due to an invalid action.yml. This can also be used as a way to verify that the action.yml is (still) valid before releasing/tagging new versions.
  • can use gh api to retrieve workflow run results
  • can view results manually in browser anywhere workflow run results are displayed (e.g. Actions tab, commit list/history, branch landing page, pr checks), or in user notifications (if configured)
  • maybe auto-open an issue or pr on failure for extra visibility

NOTE: remember that pushing commits from within gh actions/ci will not trigger further events/workflows unless pushed using a custom token/pat