implement bundle command

[cdepillabout]

purs and spago both implement a bundle command.

This command bundles all the output .js files for each PureScript module into a single .js. This single .js file is easy to deploy or copy somewhere else.

It may make sense for us to also implement a bundle command in purenix. This command would output a single .nix for the entire project. This single .nix file would be easy to copy to another project (or possibly even use in a larger repo like Nixpkgs).

We need to investigate exactly how the bundle command works in purs and spago. We need to come up with a good way of including all our output Nix code into a single file.

This was mentioned in https://github.com/purenix-org/purenix/issues/22#issuecomment-939258577.

[jonascarpay]

What does bundle do with FFI files, does it simply inline those as well?