default.nix, shell.nix: init

This commit inits the default.nix and shell.nix. They support nix build and nix-shell, respectively.

• nix-shell sets up the build environment for Veloren so that you can manually build it using cargo build. This is for developping as it speeds up subsequent compilations because intermediate compilation artifacts are reused.
• nix build builds Veloren, stores the result in the Nix store and creates a result symlink to it. Make sure that the LFS files are checked out before using nix build.