Move argument parsing

[yonaskolb]

At the moment Beak parses your function call from the CLI arguments and then embeds that function call at the end of your script and copies it into a main.swift in an executable Swift package.

It would be good to move the argument parsing code into generated package. That way it wouldn't have to be recompiled when you make a different function call (due to the source file changing). And then as it's a self contained package, we can have an install command, that will then copy the built package executable to somewhere like /usr/local/bin