Add CI checks

[krzema12]

The contents of this repo can be wrong in multiple ways:

• malformed YAML
• contents of action-types.yml not compliant with https://github.com/typesafegithub/github-actions-typing
• the action or a given version doesn't exist
• the names of the inputs/outputs don't correspond to what the action exposes
• maybe more

We'd like to have a CI check that will let the contributors add and modify the typings with confidence.

Ideally, as a stretch goal, we'd like to share the parsing logic with https://github.com/typesafegithub/github-actions-typing.