Expose the platform wrapper modules as the Nixvim configuration options
`build.nixosModule`, `build.homeModule`, and `build.nixDarwinModule`.
This makes it possible to reuse a single Nixvim configuration across
NixOS, Home Manager, and nix-darwin without re-importing modules into
`programs.nixvim` manually.
Evaluating these wrapper modules requires a "bare" Nixvim configuration;
one that does not define `pkgs` or `nixpkgs.hostPlatform`. Such a
configuration would normally fail to evaluate, but disabling
`_module.check` provides a sufficiently lazy evaluation to access the
wrapper options.
To prevent the `_module.check = false` module from leaking into user
configs, it has a unique module key and gets disabled inside the wrapper
modules (`wrappers/_shared.nix`).
