update js-client api (migrate to TS)

Summary:
JS/TS api:
- migrate to TS
- some refactoring (get rid of bridge, make client abstract)

Implementation isn't full yet, things to be implemented:
- let plugins connect on init command from Flipper
- implement Responder

Further plans:
- make fully compatible with react-native api without breaking changes

Reviewed By: mweststrate

Differential Revision: D21839377

fbshipit-source-id: 9e9fe4ad01632f958b59eb255c703c6cbc5fafe2

flipper-js-client-sdk/README.md

@@ -0,0 +1,34 @@
+# flipper-sdk-api
+
+SDK to build Flipper clients for JS based apps
+
+## Installation
+
+`yarn add flipper-client-sdk`
+
+## Usage
+
+## Example
+
+```TypeScript
+class SeaMammalPlugin extends AbsctractFlipperPlugin {
+  getId(): string {
+    return 'sea-mammals';
+  }
+
+  runInBackground(): boolean {
+    return true;
+  }
+
+  newRow(row: {id: string, url: string, title: string}) {
+    this.connection?.send("newRow", row)
+  }
+}
+
+const flipperClient = newWebviewClient();
+cosnt plugin = new SeaMammalPlugin();
+flipperClient.addPlugin();
+flipperClient.start('Example JS App');
+plugin.newRow({id: '1', title: 'Dolphin', url: 'example.com'})
+
+```