programs.zsh.syntax-highlighting: refactor highlighters option for proper validation

Right now the `programs.zsh.syntax-highlighting.highlighters` option
lacks appropriate validation which can cause confusing things when
mistyping a higlighter for zsh-syntax-highlighting.

nixos/modules/programs/zsh/zsh-syntax-highlighting.nix

@@ -18,7 +18,17 @@ in
 
         highlighters = mkOption {
           default = [ "main" ];
-          type = types.listOf(types.str);
+
+          # https://github.com/zsh-users/zsh-syntax-highlighting/blob/master/docs/highlighters.md
+          type = types.listOf(types.enum([
+            "main"
+            "brackets"
+            "pattern"
+            "cursor"
+            "root"
+            "line"
+          ]));
+
           description = ''
             Specifies the highlighters to be used by zsh-syntax-highlighting.