zsh: refactor zsh configuration for better order control over .zshrc (#6479)

* zsh: add initContent option for custom .zshrc content insertion

- Users can add content anywhere by using `lib.mkOrder`, `lib.mkBefore`
and `lib.mkAfter` custom configurations.
- Add test cases to verify the insertion of content before and after
existing configurations in `.zshrc`.
consolidate zshrc content tests into a single priorities test

modules/misc/news.nix

@@ -2129,6 +2129,17 @@ in {
           Cohere, Groq).
         '';
       }
+
+      {
+        time = "2025-03-11T02:34:43+00:00";
+        condition = config.programs.zsh.enable;
+        message = ''
+          A new module is available: 'programs.zsh.initContent'.
+
+          initContent option allows you to set the content of the zshrc file,
+          you can use `lib.mkOrder` to specify the order of the content you want to insert.
+        '';
+      }
     ];
   };
 }