Add .completion() method to ChatCraftChat class

[humphd]

We need to start untangling our chat completions logic from the React components. The first step is to extract a new method on the ChatCraftClass called completion() so that you can do this anywhere in our code (React or lib code):

await chat.completion();

To get here, my suggestion is:

• Look at the code in https://github.com/tarasglek/chatcraft.org/blob/main/src/Chat/ChatBase.tsx#L205, which is the main place where we do a chat completion when the user submits the prompt form
• Try to separate out the state logic in this code (e.g., for handling scrolling, etc.) from the actual logic for doing the chat completion. The chat.completion() method shouldn't do anything to the UI, only operate on the chat instance.

To begin, I would do this work without altering the current code in https://github.com/tarasglek/chatcraft.org/blob/main/src/Chat/ChatBase.tsx#L205. We'll do that in a follow-up.