fontconfig: set defaultFonts with mkOrder 600

Just for reference:

```nix
mkBefore = mkOrder 500;
defaultOrderPriority = 1000;
mkAfter = mkOrder 1500;
```

modules/fontconfig/fontconfig.nix

@@ -1,16 +1,17 @@
-{ mkTarget, lib, ... }:
+{ mkTarget }:
+{ lib, ... }:
 mkTarget {
   name = "fontconfig";
   humanName = "Fontconfig";
 
   configElements =
-    { fonts }:
+    { listTargetIndex, fonts }:
     {
       fonts.fontconfig.defaultFonts = lib.genAttrs [
         "monospace"
         "serif"
         "sansSerif"
         "emoji"
-      ] (family: [ fonts.${family}.name ]);
+      ] (family: lib.mkOrder listTargetIndex [ fonts.${family}.name ]);
     };
 }

modules/fontconfig/hm.nix

@@ -1,6 +1,6 @@
 { lib, mkTarget, ... }:
 {
   imports = [
-    (lib.modules.importApply ./fontconfig.nix { inherit mkTarget lib; })
+    (lib.modules.importApply ./fontconfig.nix { inherit mkTarget; })
   ];
 }

modules/fontconfig/nixos.nix

@@ -1,6 +1,6 @@
 { lib, mkTarget, ... }:
 {
   imports = [
-    (lib.modules.importApply ./fontconfig.nix { inherit mkTarget lib; })
+    (lib.modules.importApply ./fontconfig.nix { inherit mkTarget; })
   ];
 }

stylix/darwin/default.nix

@@ -17,6 +17,7 @@ in
     ../target.nix
     ../release.nix
     ../overlays.nix
+    ../ordering.nix
   ] ++ autoload;
   config.warnings =
     lib.mkIf

stylix/droid/default.nix

@@ -11,6 +11,7 @@ in
     ../pixel.nix
     ../target.nix
     ../overlays.nix
+    ../ordering.nix
   ] ++ autoload;
 
   # See https://github.com/nix-community/nix-on-droid/issues/436

stylix/hm/default.nix

@@ -20,6 +20,7 @@ in
     ../target.nix
     ../release.nix
     ../overlays.nix
+    ../ordering.nix
   ] ++ autoload;
   config.warnings =
     lib.mkIf

stylix/nixos/default.nix

@@ -19,6 +19,7 @@ in
     ../target.nix
     ../release.nix
     ../overlays.nix
+    ../ordering.nix
   ] ++ autoload;
   config.warnings =
     lib.mkIf

stylix/ordering.nix

@@ -0,0 +1,13 @@
+{ lib, ... }:
+{
+  options.stylix.listTargetIndex = lib.mkOption {
+    description = ''
+      The target index at which stylix inserts into lists.
+      Intended for use as argument to `lib.mkOrder`.
+    '';
+    type = lib.types.int;
+    default = 600;
+    internal = true;
+    readOnly = true;
+  };
+}