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/message.ts

@@ -0,0 +1,36 @@
+/**
+ * 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
+ */
+
+export interface FlipperRequest {
+  method: string;
+  params?: {
+    method: string;
+    api: string;
+    params?: unknown;
+  };
+}
+export type FlipperResponse =
+  | {
+      id: number;
+      success: object | string | number | boolean | null;
+      error?: never;
+    }
+  | {
+      id: number;
+      success?: never;
+      error: FlipperErrorMessage;
+    };
+export interface FlipperErrorMessage {
+  message: string;
+  stacktrace?: string;
+  name?: string;
+}
+export interface FlipperMessageBus {
+  sendData(data: FlipperRequest | FlipperResponse): void;
+}