nvim-lsp: add zls support

plugins/nvim-lsp/default.nix

@@ -10,6 +10,7 @@ in
     ./rust-analyzer.nix
     ./rnix-lsp.nix
     ./pyright.nix
+    ./zls.nix
   ];
 
   options = {

plugins/nvim-lsp/zls.nix

@@ -0,0 +1,18 @@
+{ config, pkgs, lib, ... }:
+with lib;
+let
+  cfg = config.programs.nixvim.plugins.lsp.servers.zls;
+in
+{
+  options = {
+    programs.nixvim.plugins.lsp.servers.zls = {
+      enable = mkEnableOption "Enable zls, for Zig.";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    programs.nixvim.extraPackages = [ pkgs.zls ];
+
+    programs.nixvim.plugins.lsp.enabledServers = [ "zls" ];
+  };
+}