Add a simple chat UI and history

[fofr]

https://github.com/replicate/chat/assets/319055/2aa45d00-01b1-45d5-82ab-ed4124cc79c3

• components for message and chat form
• styles to put chat at the bottom
• styles for user vs assistant messaging
• simple chat history passed in with prompt

[cbh123]

I may make a fork of this later on for a 7b vs 13b vs 70b playground. would be helpful for a blog post