plugin/luasnip: Add jsregexp lua dependency (#274)

The lua dependency is required to perform transforms (luasnip can
fallback to a simpler algorithm if it's absent). This requires us to
expose the `extraLuaPackages` option of makeNeovimConfig.

plugins/snippets/luasnip/default.nix

@@ -91,6 +91,7 @@ in {
   in
     mkIf cfg.enable {
       extraPlugins = [cfg.package];
+      extraLuaPackages = ps: [ps.jsregexp];
       extraConfigLua = concatStringsSep "\n" fromVscodeLoaders;
     };
 }