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

tests/modules/programs/zsh/default.nix

@@ -9,4 +9,5 @@
   zsh-prezto = ./prezto.nix;
   zsh-syntax-highlighting = ./syntax-highlighting.nix;
   zsh-abbr = ./zsh-abbr.nix;
+  zshrc-contents-priorities = ./zshrc-content-priorities.nix;
 }

tests/modules/programs/zsh/zshrc-content-priorities.nix

@@ -0,0 +1,36 @@
+{ lib, ... }: {
+  programs.zsh = {
+    enable = true;
+
+    initContent = lib.mkMerge [
+      (lib.mkBefore ''
+        # High priority (mkBefore)
+        echo "High priority content"
+      '')
+
+      (lib.mkAfter ''
+        # Low priority (mkAfter)
+        echo "Low priority content"
+      '')
+
+      ''
+        # Default priority
+        echo "Default priority content"
+      ''
+    ];
+
+    zprof.enable = true;
+  };
+
+  nmt.script = ''
+    assertFileExists home-files/.zshrc
+
+    assertFileContains home-files/.zshrc "zmodload zsh/zprof"
+    assertFileContains home-files/.zshrc "High priority content"
+    assertFileContains home-files/.zshrc "Default priority content"
+    assertFileContains home-files/.zshrc "Low priority content"
+
+    assertFileRegex home-files/.zshrc '^zmodload zsh/zprof'
+    assertFileRegex home-files/.zshrc 'echo "Low priority content"$'
+  '';
+}