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/jest.config.js

@@ -0,0 +1,15 @@
+/**
+ * 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
+ */
+
+module.exports = {
+  preset: "ts-jest",
+  clearMocks: true,
+  coverageReporters: ['json-summary', 'lcov', 'html'],
+  testMatch: ['**/**.spec.(js|jsx|ts|tsx)'],
+};