refactor: Initialize ZenThemeMarketplace and ZenThemeMarketplaceChild modules

Initialize the ZenThemeMarketplace module in ZenThemeMarketplace.mjs and the ZenThemeMarketplaceChild module in ZenThemeMarketplaceChild.sys.mjs. This commit sets up the basic structure for these modules, including empty `init` and `handleEvent` methods.

Refactor ZenThemeMarketplace.mjs:
- Add `init` method

Refactor ZenThemeMarketplaceChild.sys.mjs:
- Add `initiateThemeMarketplace` method
- Add `addIntallButton` method

src/ZenThemeMarketplace.mjs

@@ -0,0 +1,5 @@
+
+const gZenThemeMarketplace = {
+  init() {
+  },
+};

src/actors/ZenThemeMarketplaceChild.sys.mjs

@@ -0,0 +1,26 @@
+
+export class ZenThemeMarketplaceChild extends JSWindowActorChild {
+  constructor() {
+    super();
+  }
+
+  handleEvent(event) {
+    switch (event.type) {
+      case "DOMContentLoaded":
+        this.initiateThemeMarketplace();
+        break;
+      default:
+    }
+  }
+
+  initiateThemeMarketplace() {
+  }
+
+  addIntallButton() {
+    const actionsContainer = this.contentWindow.document.getElementById("theme-actions");
+    if (!actionsContainer) {
+      console.error("ZenThemeMarketplaceChild: Could not find theme-actions container");
+      return;
+    }
+  }
+};