Support using outputs as templating variables for PR and commits

[ericcornelissen]

Summary

The actions in this project now output some information (#62) about the execution which might be used in commit messages (#13, #22) or Pull Request titles/descriptions (#12). However, when using the batteries included actions the Action outputs can't be used in the commit-message/pr-description/pr-title inputs. This would be useful because it would allow for including contextual information in these fields.

Idea

I'm thinking that adopting a templating format similar to GitHub Action expressions would make sense, which would substitute outputs into the provided string.

For example, the following configuration

- uses: ericcornelissen/tool-versions-update-action/pr@v0
  with:
    max: 1
    pr-title: Bump {{updated-tools}}

would results in Pull Requests with titles such as "Bump actionlint" or "Bump shellcheck".