Implementing JSI drainMicrotasks

Summary:
This diff implement the new JSI API `drainMicrotasks` by draining
the internal job queue and throw JSIException.

Reviewed By: dulinriley

Differential Revision: D27731602

fbshipit-source-id: b2c99e516bd183df37778b25d4202b1e0dd077eb

API/hermes/hermes.cpp

@@ -1473,6 +1473,11 @@ jsi::Value HermesRuntimeImpl::evaluateJavaScript(
 }
 
 bool HermesRuntimeImpl::drainMicrotasks(int maxMicrotasksHint) {
+  if (runtime_.useJobQueue()) {
+    checkStatus(runtime_.drainJobs());
+  }
+  // \c drainJobs is currently an unbounded execution, hence no exceptions
+  // implies drained until TODO(T89426441): \c maxMicrotasksHint is supported
   return true;
 }