Add Firebase AI SDK sample app

[dlarocque]

Adds a sample app for the Firebase AI SDK built with React.

Features

• Backend Toggle: Switch between GoogleAIBackend and VertexAIBackend with a toggle
• Chat: Streaming, history, function calling, structured output, multimodal input, token counting, parameter controls
• Imagen: Image generation with parameter controls

[hsubox76]

Create a top level README.md (ai/README.md) with a more general intro "This directory contains code samples for the Firebase AI SDK for Web" and a table of contents that I guess currently contains only one entry - the react sample app (you can add more later).

[hsubox76]

This is really great and comprehensive. We should definitely add simplified plain JS single-case examples later for people who are in a hurry, or Node or non-React users, but not urgent.