Add fetchWorker for fetching off the main thread

src/workers/fetch.test.ts

@@ -0,0 +1,14 @@
+import { assert } from '@/deps-test.ts';
+
+import { fetchWorker } from './fetch.ts';
+
+Deno.test('fetchWorker', async () => {
+  await sleep(2000);
+  const response = await fetchWorker('https://example.com');
+  const text = await response.text();
+  assert(text.includes('Example Domain'));
+});
+
+function sleep(ms: number) {
+  return new Promise((resolve) => setTimeout(resolve, ms));
+}