[3.14] gh-124621: Emscripten: Add support for async input devices (GH-136822) (GH-136935)

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`.
(cherry picked from commit 7ae4749d06)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>

Lib/test/test_capi/test_emscripten.py

@@ -0,0 +1,25 @@
+import unittest
+from test.support import is_emscripten
+
+if not is_emscripten:
+    raise unittest.SkipTest("Emscripten-only test")
+
+from _testinternalcapi import emscripten_set_up_async_input_device
+from pathlib import Path
+
+
+class EmscriptenAsyncInputDeviceTest(unittest.TestCase):
+    def test_emscripten_async_input_device(self):
+        jspi_supported = emscripten_set_up_async_input_device()
+        p = Path("/dev/blah")
+        self.addCleanup(p.unlink)
+        if not jspi_supported:
+            with open(p, "r") as f:
+                self.assertRaises(OSError, f.readline)
+            return
+
+        with open(p, "r") as f:
+            for _ in range(10):
+                self.assertEqual(f.readline().strip(), "ab")
+                self.assertEqual(f.readline().strip(), "fi")
+                self.assertEqual(f.readline().strip(), "xy")