Typings for individual Decoder factory (#38631)

Decoder can be built standalone (is part of the actual public interface
and part of the performance-test interface described in the README.md of
the project). Moreover, Decoder is an event emitter. The interface has
been update accordingly.

types/msgpack-lite/index.d.ts

@@ -5,6 +5,7 @@
 
 /// <reference types="node" />
 import * as stream from 'stream';
+import { EventEmitter } from 'events';
 
 /**
  * encode from JS Object to MessagePack
@@ -84,7 +85,9 @@ export interface Encoder {
   end(chunk: any): void;
 }
 
-export interface Decoder {
+export function Decoder(options?: DecoderOptions): Decoder;
+
+export interface Decoder extends EventEmitter {
   bufferish: any;
   offset: number;
   fetch(): void;

types/msgpack-lite/msgpack-lite-tests.ts

@@ -84,3 +84,18 @@ function customExtensionTypes() {
     return new MyVector(array[0], array[1]); // return Object deserialized
   }
 }
+
+// https://github.com/kawanet/msgpack-lite#benchmarks
+// (this is not well documented, but in the test there is an example usage.)
+function standaloneDecoder() {
+  const decoder = msgpack.Decoder();
+  const object = { test: "Object" };
+  const encoded = msgpack.encode(object);
+  decoder.on('data', (obj) => {
+    if (object.test !== obj.test) {
+      throw Error();
+    }
+  });
+
+  decoder.push(encoded);
+}