nix: move package to nix/package.nix and add expose overlay

[LucasOe]

This pull request refactors the Nix packaging setup.

• Cleaned up flake.nix and moved the package derivation logic into a new file nix/package.nix
• Added an overlay output to expose the package as scopebuddy
• The package now uses lib.cleanSource to avoid including unnecessary files in the build context
• Added a meta section

Why add an overlay?

The overlay makes it possible for users to build scopebuddy with their own customized package set.

A user may override gamescope to apply a patch or workaround. When installing via the flake's packages output, those overrides are not used. Installing via the overlay (pkgs.scopebuddy) ensures user-defined overrides are respected.

For example, some users may want to override gamescope to work around upstream issues:

{
  nixpkgs.overlays = [
    (final: prev: {
      # https://github.com/ValveSoftware/gamescope/issues/1924
      gamescope = prev.gamescope.overrideAttrs (_: {
        NIX_CFLAGS_COMPILE = [ "-fno-fast-math" ];
      });
    })
  ];
}