[skip ci] Add queueMicrotask method to JSI

Summary:
Changelog: [internal]

## Context

Microtasks are an important aspect of JavaScript and they will become increasingly important in the hosts where we're currently using JSI.

For example, React Native is going to adopt an event loop processing model similar to the one on the Web, which means it would need the ability to schedule and execute microtasks in every iteration of the loop. See react-native-community/discussions-and-proposals#744 for details.

JSI already has a method to execute all pending microtasks (`drainMicrotasks`) but without a method to schedule microtasks this is incomplete.

We're currently testing microtasks with Hermes using an internal method to schedule microtasks (`HermesInternal.enqueueJob`) but we need a method in JSI so this also works in other runtimes like JSC and V8.

## Changes

This adds the `queueMicrotask` to the Runtime API in JSI so we have symmetric API for microtasks and we can implement the necessary functionality.

The expectation for JSI implementations is to queue microtasks from this method and from built-ins like Promises and async functions in the same queue, and not drain that queue until explicitly done via `drainMicrotasks` in JSI.

This also modifies Hermes and JSC to provide stubs for those methods, and the actual implementation will be done in following diffs.

Differential Revision: D54302536

packages/react-native/ReactCommon/jsc/JSCRuntime.cpp

@@ -51,6 +51,7 @@ class JSCRuntime : public jsi::Runtime {
       const std::shared_ptr<const jsi::Buffer>& buffer,
       const std::string& sourceURL) override;
 
+  void queueMicrotask(const jsi::Function& callback) override;
   bool drainMicrotasks(int maxMicrotasksHint = -1) override;
 
   jsi::Object global() override;
@@ -434,7 +435,9 @@ jsi::Value JSCRuntime::evaluateJavaScript(
   return createValue(res);
 }
 
-bool JSCRuntime::drainMicrotasks(int maxMicrotasksHint) {
+void JSCRuntime::queueMicrotask(const jsi::Function& /*callback*/) {}
+
+bool JSCRuntime::drainMicrotasks(int /*maxMicrotasksHint*/) {
   return true;
 }
 

packages/react-native/ReactCommon/jsi/jsi/decorator.h

@@ -126,6 +126,9 @@ class RuntimeDecorator : public Base, private jsi::Instrumentation {
       const std::shared_ptr<const PreparedJavaScript>& js) override {
     return plain().evaluatePreparedJavaScript(js);
   }
+  void queueMicrotask(const jsi::Function& callback) override {
+    return plain().queueMicrotask(callback);
+  }
   bool drainMicrotasks(int maxMicrotasksHint) override {
     return plain().drainMicrotasks(maxMicrotasksHint);
   }
@@ -544,6 +547,10 @@ class WithRuntimeDecorator : public RuntimeDecorator<Plain, Base> {
     Around around{with_};
     return RD::evaluatePreparedJavaScript(js);
   }
+  void queueMicrotask(const Function& callback) override {
+    Around around{with_};
+    RD::queueMicrotask(callback);
+  }
   bool drainMicrotasks(int maxMicrotasksHint) override {
     Around around{with_};
     return RD::drainMicrotasks(maxMicrotasksHint);

packages/react-native/ReactCommon/jsi/jsi/jsi.h

@@ -209,6 +209,11 @@ class JSI_EXPORT Runtime {
   virtual Value evaluatePreparedJavaScript(
       const std::shared_ptr<const PreparedJavaScript>& js) = 0;
 
+  // Queues a microtask in the JavaScript VM internal Microtask (a.k.a. Job in
+  // ECMA262) queue, to be executed when the host drains microtasks in
+  // its event loop implementation.
+  virtual void queueMicrotask(const jsi::Function& callback) = 0;
+
   /// Drain the JavaScript VM internal Microtask (a.k.a. Job in ECMA262) queue.
   ///
   /// \param maxMicrotasksHint a hint to tell an implementation that it should