We should support cloud_firestore on Flutter for web.

-- Tasks --

• Move cloud_firestore to its own subdirectory to allow for federated implementations
• Add cloud_firestore_platform_interface package, with the core interface that the plugin implements, and the MethodChannel implementation
• Port cloud_firestore to use the interface defined above
• Add cloud_firestore_web implementing the interface, for web
• Endorse cloud_firestore_web from cloud_firestore as the official web platform implementation.