Fix the program being unstoppable when waiting for stdin

If we send a ‘stopPython’ message which the web worker is in the
while (true) loop, it won’t handle the message until after the loop
exits, so we need to use Pyodide’s interruptBuffer method for checking
if we’ve stopped rather than just checking the ‘stopped’ global.

pyodide/webworker.js

@@ -169,7 +169,7 @@ const readFromStdin = (bufferToWrite) => {
   postMessage({ method: "handleInput" });
 
   while (true) {
-    checkIfStopped();
+    pyodide.checkInterrupt();
     const result = Atomics.wait(stdinBuffer, 0, previousLength, 100);
     if (result === "not-equal") { break; }
   }