Implement JS flipper client

Summary:
Standardize WS implementation for JS environments.

Why do we need a separate server implementation for browsers?
Browser targets cannot authenticate via the default certificate exchange flow. We need a dedicated client for them that works over an insecure channel (without the cert exchange).

Major changes:
1. Renamed `flipper-js-client-sdk` to `js-flipper` for consistency with `react-native-flipper`
2. Updated `js-flipper` implementation to match our other existing clients

Documentation will be updated in a separate subsequent PR.

https://fb.quip.com/2mboA0xbgoxl

Reviewed By: mweststrate

Differential Revision: D31688105

fbshipit-source-id: 418aa80e0fd86361c089cf54b0d44a8b4f748efa

js/js-flipper/src/responder.ts

@@ -0,0 +1,29 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {FlipperErrorMessage, FlipperMessageBus} from './message';
+import {FlipperPluginReceiverRes} from './plugin';
+
+export class FlipperResponder {
+  constructor(
+    public readonly responderId: number,
+    private client: FlipperMessageBus,
+  ) {}
+
+  success(response?: FlipperPluginReceiverRes) {
+    this.client.sendData({
+      id: this.responderId,
+      success: response == null ? null : response,
+    });
+  }
+
+  error(response: FlipperErrorMessage) {
+    this.client.sendData({id: this.responderId, error: response});
+  }
+}