Add basic CompletionMapping and runEngine code

src/types/engine262-stubs.ts

@@ -0,0 +1,14 @@
+export enum CompletionTypes {
+  Normal = "normal",
+  Return = "return",
+  Break = "break",
+  Continue = "continue",
+  Throw = "throw",
+}
+
+export interface CompletionRecord {
+  Type: CompletionTypes;
+  Value: any;
+  // Target: Actually undefined or StringValue
+  Target: any;
+}