Lets a worker take objects through postMessage(data, transfer), so @nativescript/canvas can send an OffscreenCanvas to a worker (NativeScript/canvas#179), and fixes what kept native addons from working in workers.

Transfer lists

• postMessage(data, transfer) (or { transfer }) on Worker and in the worker. The value is cloned and the list taken when postMessage is called, as on the web, so a value that can't be cloned now throws there in the worker too, instead of becoming a messageerror on the main thread.
• ArrayBuffers in the list are copied, then detached.
• Other objects need a type registered in each isolate with __nsRegisterTransferable(name, { test, detach, attach, release }). detach turns the object into a number or string token, and the receiving isolate's handler of the same name turns it back with attach. When it can't (nothing registered there), the rest are released and the receiver gets a messageerror.
• Duplicates, detached buffers and unregistered objects throw DataCloneError; a transfer list that isn't an array throws TypeError.

Native addons in workers

• Node-API jobs: threadsafe-function calls and async-work completions run on the thread whose env made them. A worker's all ran on the UI thread, so its addons crashed (e.g. napi_create_external_arraybuffer on the worker's env, from the UI thread) or never heard back.
• Teardown: a terminating worker runs its addons' cleanup hooks and frees their envs. External ArrayBuffers still alive are finalized during env teardown, as Node does, since their backing stores go later with the isolate's heap. An external ArrayBuffer made without a finalizer had a null deleter, which crashed when a worker's isolate was disposed.

Workers

• requestAnimationFrame works in workers. Each UI frame is handed to the workers that asked for one; a worker not handed one within a frame's time (the compositor stops raising frames while nothing on screen changes) runs one itself.
• Worker paths: new Worker('~/chunk.js'), as NativeScript's webpack worker loader emits it, resolves against the app directory. It looked for app/~/chunk.js, so bundled workers didn't load.
• Worker.postMessage no longer blocks the UI thread for up to 500 ms waiting for a reply under a XAML host: replies arrive through the dispatcher. Hosts whose dispatcher only runs when they pump messages (console hosts, the unit-test harness) still wait.

Tests

• New: worker_transfer_list_moves_buffers_and_registered_objects, worker_transfer_list_rejects_what_it_cannot_take. cargo test -p runtime --lib worker passes (13), as do the integration tests.
• In @nativescript/canvas's demo app (OffscreenCanvas in Workers canvas#179): the OffscreenCanvas suites (69 tests) pass, including drawing from workers into views, requestAnimationFrame, WebGPU and terminating a drawing worker.