zsh: fix highlighter backwards compatibility

Use the module system to set the default at user prio so that existing
configs don't clear out `main`

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

modules/programs/zsh/default.nix

@@ -44,11 +44,15 @@ in
 
           highlighters = mkOption {
             type = types.listOf types.str;
-            default = [ "main" ];
+            default = [ ];
+            defaultText = ''[ "main" ]'';
             example = [ "brackets" ];
             description = ''
               Highlighters to enable
               See the list of highlighters: <https://github.com/zsh-users/zsh-syntax-highlighting/blob/master/docs/highlighters.md>
+
+              Note: The "main" highlighter is always included automatically.
+              If you'd like to exclude it, please configure with a higher priority using `mkForce`.
             '';
           };
 
@@ -476,6 +480,10 @@ in
         {
           home.packages = [ cfg.package ] ++ lib.optional cfg.enableCompletion pkgs.nix-zsh-completions;
 
+          # NOTE: Always include "main" highlighter with normal priority.
+          # Option default priority will cause `main` to get dropped by customization.
+          programs.zsh.syntaxHighlighting.highlighters = lib.mkIf cfg.syntaxHighlighting.enable [ "main" ];
+
           programs.zsh.initContent = lib.mkMerge [
             (mkOrder 510 "typeset -U path cdpath fpath manpath")
 

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

@@ -7,7 +7,6 @@
       enable = true;
       package = pkgs.hello;
       highlighters = [
-        "main"
         "brackets"
         "pattern"
         "cursor"
@@ -19,7 +18,7 @@
 
   nmt.script = ''
     assertFileContains home-files/.zshrc "source ${pkgs.hello}/share/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh"
-    assertFileContains home-files/.zshrc "ZSH_HIGHLIGHT_HIGHLIGHTERS=(main brackets pattern cursor)"
+    assertFileContains home-files/.zshrc "ZSH_HIGHLIGHT_HIGHLIGHTERS=(brackets pattern cursor main)"
     assertFileContains home-files/.zshrc "ZSH_HIGHLIGHT_STYLES[comment]='fg=#6c6c6c'"
   '';
 }