Validate `github.event.*` properties about actionlint

[rhysd]

Good point. actionlint currently types github.event as an object which accepts any properties.

It's possible to give stricter type to github.event. But it is a bit challenging in the following points:

• there are so many properties under github.event. Crafting the type definition is a bit hard.
• The event payloads are depending on GitHub API specification, meant that they would change at some point in the future.

My idea is preparing a script to generate a Go code for the type of github.event and generating a code regularly. We already do this for popular_actions.go and all_webhooks.go. I think we can do the similar thing for the type definition of github.event payload.

Octokit maintains payloads information at https://github.com/octokit/webhooks. We might be able to use it.

[rhysd]

Properties other than github.event.* are already typed strictly following the official document (e.g. runner.tmp). So I believe the remaining payloads which are not typed well are only github.event.*. Let me update the title more specifically.