* ghostty: allow darwin users to manager their config
Currently nixpkgs does not contain a package defintion for ghostty
compatible with Darwin. Darwin users may still want to use this module
to manage their config or share config between systems. This carries
over behaviour from the beta period where this same technique was used.
see: 887e13a6e7/module.nix (L167-L173)
Also improves validation to cover theme files.
* ghostty: guard all package access with isLinux
ghostty is currently marked as broken in nixpkgs. Darwin users still
want to manage ghostty config via home-manager. Avoiding installing the
package and any extra files that depend on the package outside Linux
allows this.
* ghostty: allow nullable
