Parallel OxiPNG improvements

- Refactor to work around Chromium's issue with postMessage queuing. https://bugs.chromium.org/p/chromium/issues/detail?id=1075645
 - Convert codec code to TypeScript.
 - Make separate parallel and non-parallel builds.
 - Switch to nightly Rust for OxiPNG to allow parallel builds (but also reuse it for regular builds to avoid installing two toolchains).

codecs/oxipng/worker.ts

@@ -0,0 +1,20 @@
+/// <reference lib="webworker" />
+
+import initOxiPNG, { start_worker_thread } from './pkg-parallel';
+
+export type WorkerInit = [WebAssembly.Module, WebAssembly.Memory];
+
+addEventListener(
+  'message',
+  async (event) => {
+    // Tell the "main" thread that we've received the message.
+    //
+    // At this point, the "main" thread can run Wasm that
+    // will synchronously block waiting on other atomics.
+    postMessage(null);
+
+    await initOxiPNG(...(event.data as WorkerInit));
+    start_worker_thread();
+  },
+  { once: true },
+);