v0.1

1. Automatically read Lean version from lean-toolchain
2. Load multiple lean4 versions at the same time is now possible with overlays
3. lake2nix: Convert a lake-manifest.json into Nix expressions. There's a template example for building a project with the aesop dependency

Currently the only supported Lean major version is v4.12.0 since that is the version for which the nix flake was deprecated.