Support chatops-style release skipping with PRs

[ericcornelissen]

Relates to #24

Summary

Add support for a way to tell the /pr variant of the Action to not suggest updating a tool to a given version. In particular, when it creates a PR to upgrade a tool to a certain version, it should be possible to prevent the Action from updating that tool to that version in the future without having to change the configuration in the workflow.

A classic chatops way of achieve thins would be support for a comment like @tool-version-update-action-bot ignore this version (which would then close the PR and prevent it from opening the same PR in the future). Though I would be okay with that, it would probably be significantly easier to support a workflow where users can just close the PR and then it won't be recreated (in which case those instructions should be added to the default PR description).

[ericcornelissen]

It might make sense to use a similar approach as suggested here in the Dependabot docs, specifically:

You can check whether a repository has stored ignore preferences by searching the repository for "@dependabot ignore" in:comments, [...]

[ericcornelissen]

See also https://github.com/marketplace/actions/chatops-for-pull-requests