Add a prepare + compile command

[CGA1123]

prepare and compile are separate commands at the moment, should there be a combined one so it's a fire and forget job?

slugcmplr run [APP] --build-dir [BUILD-DIR] --source-dir [SOURCE-DIR] --cache-dir [CACHE-dir]

[CGA1123]

I think this should probably be done, and I kinda want to extract an upload command out of compile. 🤔

Really there should be a version of prepare that doesn't need a Heroku application. It can be given a --stack, multiple --buildpack, and a --source-dir

Not 100% sure on that though... maybe it doesn't need a different CLI entrypoint, but should be kept separate internally (?)