LibWeb: Prevent crash when loading module in worker

The import map is defined for all global objects, not just the window.

Tests/LibWeb/Text/input/Worker/worker.mjs

@@ -1,3 +1,5 @@
+import "./worker.mjs";
+
 self.onmessage = ({ data }) => {
     self.postMessage(`Worker responding to: ${data}`);
 };