Use the forAllSystems pattern to reduce duplication and inline the
NixOS module directly in nixosSystem call. This eliminates the need for
separate nixosModules and nixosConfigurations outputs, making the
template more concise and easier to understand.
Also use virtualisation.munix namespace for the option.
Add a NixOS option to configure the default command for the VM and
provide a system.build.munix output that wraps munix with the correct
toplevel and default command. This reduces boilerplate in downstream
flakes since they no longer need to manually wrap munix.
The template now uses these new features, significantly simplifying
the apps definition.
Replace inline flake.nix example with a proper flake template that users
can instantiate with `nix flake init`. This makes it easier to get
started and we can easier test the example.
