Get basic test up and passing

This replaces the 'ws' library with built-in websockets so that we can mock a server and successfully connect to it. A simple test verifies that Unit is handling a message request and response as we expect it to.

esm/unit.d.ts

@@ -30,6 +30,8 @@ export declare class Unit extends EventEmitter {
      * Connects to the specified unit and maintains a connection to it until `close()` is called.
      */
     connect(): Promise<void>;
+    private onOpen;
+    private onError;
     /**
      * Closes the connection to the unit.
      */