Initial keycloak.js bindings

Changelog

• 3 constructing flavours for KeycloackInstance
• Future API to replace JS Promise
• Registers callback with Dart function
• Auto Generation of JS Interop file
• Simple script that utilize js_facade_gen
• Instructions in Readme to repair the generated file for usage

Example

• Showcases most of the Keycloak functionalities
• Provides sample code to most situation