Add websocketServer type (#51)

* Add websocketServer type

* Add TS test for app.websocketServer

Author: GuskiS

index.d.ts

@@ -40,6 +40,8 @@ declare module 'fastify' {
       >,
       handler?: WebsocketHandler<HttpRequest, HttpResponse>
     ): FastifyInstance<HttpServer, HttpRequest, HttpResponse>;
+    
+    websocketServer: WebSocket.Server;
   }
 
   interface RouteOptions<

test/types/index.test-d.ts

@@ -3,6 +3,7 @@ const fastify = require('fastify');
 import { SocketStream } from '../..';
 import { WebsocketHandler, FastifyRequest, FastifyInstance } from 'fastify';
 import { expectType } from 'tsd';
+import { Server } from 'ws';
 
 const app: FastifyInstance = fastify();
 app.register(wsPlugin);
@@ -13,6 +14,7 @@ const handler: WebsocketHandler = (
   params
 ) => {
   expectType<SocketStream>(connection);
+  expectType<Server>(app.websocketServer);
   expectType<{ [key: string]: any } | undefined>(params);
 };