feat(VirtualTestScheduler): implements initial VirtualTestScheduler interface

Author: kwonoj

src/VirtualTestScheduler.ts

@@ -0,0 +1,24 @@
+import { VirtualTimeScheduler } from 'rxjs/scheduler/VirtualTimeScheduler';
+
+/**
+ * Configuration options for scheduler instance.
+ *
+ */
+export interface SchedulerStartOptions {
+  /**
+   * Represents virtual time when scheduler instance is created, start to watch observables
+   */
+  created: number;
+
+  subscribed: number;
+  unsubscribed: number;
+}
+
+/**
+ * Exposes interfaces to control scheduler runs based on virtual time frame,
+ * allows to assert / inspect observables synchronously
+ *
+ */
+export interface VirtualTestScheduler extends VirtualTimeScheduler {
+  readonly isFlushing: boolean;
+}