[guybedford]

Adds the _callback variant alongside the _promise variant of #27880, so the result of an asynchronous JS library function can be delivered to a C callback on the calling thread without a promise handle.

foo_callback(args..., callback, arg), listed as __async: ['callback'], runs the same body (with sync always false) and calls callback(err, result, arg) once the result is available (NULL for a void function), with err set (and result NULL) if the body's Promise rejects or the main thread exits before completing it. Delivery is always after the call has returned, including for a synchronous completion, and a pending callback holds the runtime alive like a timer. On the main thread the body's value or Promise is settled with .then under callUserCallback. From a pthread it is emscripten_proxy_callback_with_ctx of the main-thread body: the result rides in the proxied call's arg as for _promise, and the callback runs as a task on the pthread's proxying queue, so no Promise object or handle exists on that thread and a C event loop can collect it from its own queue processing. As with _promise, the variant is emitted only where used.

void lookup_callback(const char* name,
                     void (*callback)(bool err, void* result, void* arg),
                     void* arg);

Tests: jslib.test_jslib_async_auto_variants_callback in plain, ASYNCIFY, JSPI and PROXY_TO_PTHREAD builds, wasm32 and wasm64.

Made with AI assistance under my review