Switch CLI to subcommands

[Profpatsch]

Currently there are two “subcommands”:

• yarn2nix [./yarn.lock]: converts the lockfile to nix code from stdout
• yarn2nix --template [./package.json]: converts the package.json to a compatible bit of metadata

We are gonna have more stuff like bundle, so we should do a (breaking) switch to subcommands now that we still can.

• yarn2nix convert yarn-lock [path/to/yarn.lock]
• yarn2nix convert package-json [path/to/package.json]

should give us a good first namespace for the existing (low-level) functionality, invoking yarn2nix on its own should be the same as yarn2nix --help.