zellij: Add extraConfig

This allows working around [#4659](https://github.com/nix-community/home-manager/issues/4659)

While fixing `toKDL` might be fun, I just don't think it's a good use of
anyone's time.

---

This is still untested.

tests/modules/programs/zellij/config.nix

@@ -0,0 +1,37 @@
+{ lib, ... }:
+
+{
+  programs = {
+    zellij = {
+      enable = true;
+
+      settings = {
+        default_layout = "welcome";
+      };
+      extraConfig = ''
+        This_could_have_been_json {
+        }
+      '';
+    };
+  };
+
+  test.stubs = {
+    zellij = { };
+  };
+
+  nmt.script = ''
+    assertFileExists home-files/.config/zellij/config.kdl
+
+    assertFileContains \
+      home-files/.config/zellij/config.kdl \
+      'default_layout "welcome"'
+
+    assertFileContains \
+      home-files/.config/zellij/config.kdl \
+      '// extraConfig'
+
+    assertFileContains \
+      home-files/.config/zellij/config.kdl \
+      'This_could_have_been_json'
+  '';
+}

tests/modules/programs/zellij/default.nix

@@ -1,4 +1,5 @@
 {
+  zellij-config = ./config.nix;
   zellij-enable-shells = ./enable-shells.nix;
   zellij-layout = ./layout.nix;
   zellij-theme = ./theme.nix;