Add action API typing

[krzema12]

Hi!

Would you be open for introducing typing for your action's API (inputs and outputs) using https://github.com/krzema12/github-actions-typing/? To see how such typing could look like, see https://github.com/krzema12/github-workflows-kt/blob/main/actions/softprops/action-gh-release/v1/action-types.yml.

Some context: I maintain https://github.com/krzema12/github-workflows-kt, it's a library for describing GitHub Actions workflows in a type-safe way, in Kotlin. Since actions themselves don't contain information about types of their input and output, we need to keep the typings in our repository to generate type-safe Kotlin code. To better scale across the number of actions, I came up with the idea of https://github.com/krzema12/github-actions-typing/ which can be adopted by library authors and the authors themselves can decide about proper typing, and thanks to this we don't have to maintain the typings ourselves. The typings are language-agnostic, I can imagine someone creating a similar library with TypeScript code generation.

Feel free to ask any questions. I'm also keen to open a PR.

Best,
Piotr

[krzema12]

@softprops do you need any explanation on this? Happy to answer questions and accept feedback.