gh-124621: Emscripten: Add support for async input devices (GH-136822)

This is useful for implementing proper `input()`. It requires the
JavaScript engine to support the wasm JSPI spec which is now stage 4.
It is supported on Chrome since version 137 and on Firefox and node
behind a flag.

We override the `__wasi_fd_read()` syscall with our own variant that
checks for a readAsync operation. If it has it, we use our own async
variant of `fd_read()`, otherwise we use the original `fd_read()`.
We also add a variant of `FS.createDevice()` called
`FS.createAsyncInputDevice()`.

Finally, if JSPI is available, we wrap the `main()` symbol with
`WebAssembly.promising()` so that we can stack switch from `fd_read()`.
If JSPI is not available, attempting to read from an AsyncInputDevice
will raise an `OSError`.

Modules/_testinternalcapi.c

@@ -2345,6 +2345,37 @@ incref_decref_delayed(PyObject *self, PyObject *op)
     Py_RETURN_NONE;
 }
 
+#ifdef __EMSCRIPTEN__
+#include "emscripten.h"
+
+EM_JS(int, emscripten_set_up_async_input_device_js, (void), {
+    let idx = 0;
+    const encoder = new TextEncoder();
+    const bufs = [
+        encoder.encode("ab\n"),
+        encoder.encode("fi\n"),
+        encoder.encode("xy\n"),
+    ];
+    function sleep(t) {
+        return new Promise(res => setTimeout(res, t));
+    }
+    FS.createAsyncInputDevice("/dev", "blah", async () => {
+        await sleep(5);
+        return bufs[(idx ++) % 3];
+    });
+    return !!WebAssembly.promising;
+});
+
+static PyObject *
+emscripten_set_up_async_input_device(PyObject *self, PyObject *Py_UNUSED(ignored)) {
+    if (emscripten_set_up_async_input_device_js()) {
+        Py_RETURN_TRUE;
+    } else {
+        Py_RETURN_FALSE;
+    }
+}
+#endif
+
 static PyMethodDef module_functions[] = {
     {"get_configs", get_configs, METH_NOARGS},
     {"get_recursion_depth", get_recursion_depth, METH_NOARGS},
@@ -2447,6 +2478,9 @@ static PyMethodDef module_functions[] = {
     {"is_static_immortal", is_static_immortal, METH_O},
     {"incref_decref_delayed", incref_decref_delayed, METH_O},
     GET_NEXT_DICT_KEYS_VERSION_METHODDEF
+#ifdef __EMSCRIPTEN__
+    {"emscripten_set_up_async_input_device", emscripten_set_up_async_input_device, METH_NOARGS},
+#endif
     {NULL, NULL} /* sentinel */
 };