A `checkout` action similar to `actions/checkout`

[Byron]

Summary 💡

The gix-powered checkout action should be serving as drop-in for the default usage of actions/checkout, so people are able to simply replace actions/ with owner-or-organization to benefit from the increased performance and efficiency that gix provides.

Thoughts

• When an MVP is ready and it's genuinely useful already, the repository could be moved into an official location.
• The MVP should be supporting the most basic usage of actions/checkout that most people are using. Parameters are secondary, but unsupported ones should already trigger failure stating that it's not implemented, if it's not implemented.

Motivation 🔦

Leverage the superior cloning and checkout performance of gix to make CI a little faster, probably while heating the data-center less.