Add action typings

[krzema12]

Closes #591.

This change introduces typings for the action's API using https://github.com/typesafegithub/github-actions-typing, along with a workflow that cross-validates them with action.yml. It helps both humans and machines reason about the API of the action in a standardized, readable way. You can also think of it as an additional linter or documentation tool.

Adoption notes:

• this mechanism is already fully functional, and is used by approx. 30 actions: https://github.com/typesafegithub/github-actions-typing/network/dependents
• it currently uses Kotlin and its JVM target, and building the docker image every time the action is run is surplus. If we decide to introduce the typings here in this template, I'm open to changing the approach to make it less resource-intensive, e.g. by prebuilding the Docker image, using JS runtime, or yet another way

Testing done

Checked in my fork that the new workflow succeeds, see https://github.com/krzema12/typescript-action/actions/runs/6022936242/job/16338691428

[ncalteen]

Hi @krzema12, thanks for introducing me to typesafegithub/github-actions-typing! This is an interesting project :)

At this time, we're not going to introduce this into the template repository. We do not want to add in third party actions because this can affect the ability of GitHub Enterprise Cloud users to make use of this repository. For example, administrators may enforce a policy restricting the types of third-party actions that may be run on repositories in the enterprise.

[krzema12]

@ncalteen have you considered introducing typings as a first-party feature that would be checked by GitHub itself, without auxiliary actions?

[ncalteen]

I like the idea! I will check about submitting this as a feature request :)

[krzema12]

@ncalteen would be really cool! I've opened this thread some time ago, with not much traction: https://github.com/orgs/community/discussions/32054