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/spawn.ts

@@ -0,0 +1,28 @@
+import initOxiPNG, {
+  worker_initializer,
+  start_main_thread,
+  optimise,
+} from './pkg-parallel';
+import wasmUrl from './pkg-parallel/oxipng_bg.wasm';
+import { WorkerInit } from './worker';
+
+function initWorker(worker: Worker, workerInit: WorkerInit) {
+  return new Promise((resolve) => {
+    worker.postMessage(workerInit);
+    worker.addEventListener('message', () => resolve(), { once: true });
+  });
+}
+
+async function startMainThread() {
+  const num = navigator.hardwareConcurrency;
+  const workers = Array.from({ length: num }, () => new Worker('./worker', { type: 'module' }));
+  await initOxiPNG(fetch(wasmUrl), undefined as any);
+  const workerInit: WorkerInit = worker_initializer();
+  await Promise.all(workers.map(worker => initWorker(worker, workerInit)));
+  start_main_thread(num);
+  return {
+    optimise,
+  };
+}
+
+export default startMainThread();