Remove the use of actions/checkout

[JamesIves]

I wanted to start the discussion of potentially removing the usage of actions/checkout as a dependency. I think while actions/checkout is maintained by GitHub themselves, an MVP is easily replicated and will reduce a lot of confusion when creating a workflow. All you'd need to do is add this action.

I believe the API additions would need to be as so:

• CHECKOUT: Toggles the internal checkout process on/off. I think this should be optional to turn off if people have complex workflows that require more features. This could be eventually deprecated.

• LFS: This is already a toggle, but the necessary fetch/setup commands could be run as a result of this if toggled.

• SUBMODULES: Runs the necessary setup commands to download submodules.

There would also be a way to run any pre-deploy commands. As right now you can add a step between checkout and deployment we'd need to preserve this behavior for things like yarn build. Perhaps an EXECUTE argument would suffice within a try/catch block.

If you have any thoughts on this please feel free to share them! This is just a suggestion/idea right now, and may not be implemented depending on the scope.