dev: setup flake

Set up a nix flake using crane.

• Use nix build . to build the project. The resulting binary is found under ./result/bin
  • If the build fails with an error that Cargo.lock needs to be updated, run cargo update to update all dependencies.
• Use nix flake check to run all checks. This includes:
  • Unit tests
  • formatting
  • dependency auditing
  • license checks
  • documentation tests
  • clippy checks
• Use nix develop to spawn a dev shell
  • alternatively, direnv support is given using an .envrc