feat: Adds a github formatter

[isaacsanders]

I would am working to create a github actions formatter for this library, and the first step I wanted to take is to extract the other formats into modules.

[jeremyjh]

Hi, I'm not opposed to the refactoring but can you explain a bit more about what you are trying to accomplish? Is it returning the results in a structure understood by github actions so they appear better in PRs? And then you'll do another PR to implement an action formatter?

[isaacsanders]

https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#setting-an-error-message

I have the action formatter written already, but I figured doing changes in smaller chunks was a better approach.

The link above outlines the format. It allows for inline annotations of the files from the github check run, i believe.

[jeremyjh]

@isaacsanders Makes sense - I'd say though let's just include it in this PR so that it is a complete feature.

[isaacsanders]

Added in. Thanks for the review and promptness!

[jeremyjh]

This all looks pretty good, could I ask you though to also update the mix help & readme docs to document the new "github" option?

[isaacsanders]

I've added the docs

[jeremyjh]

Looks great, thank you!

[jeremyjh]

@isaacsanders I just realized I had to release it to run CI, there are a couple of compile warnings can you please check?

[jeremyjh]

I've published to hex as 1.2.

[isaacsanders]

Thanks Jeremy! For this and for being a maintainer!