Ensure browser supports workers-in-workers (#1325)

Fixes #1324

src/worker-shared/supports-wasm-threads.ts

@@ -0,0 +1,17 @@
+import { threads } from 'wasm-feature-detect';
+
+export default async function checkThreadsSupport() {
+  const supportsWasmThreads = await threads();
+  if (!supportsWasmThreads) return false;
+
+  // Safari 16 shipped with WASM threads support, but it didn't ship with nested workers support.
+  // This meant Squoosh failed in Safari 16, since we call our wasm from inside a worker to begin with.
+
+  // Right now, this check is only run from a worker.
+  // More implementation is needed to run it from a page.
+  if (!('importScripts' in self)) {
+    throw Error('Not implemented');
+  }
+
+  return 'Worker' in self;
+}