feat(chat): adding to chat initializes buffer if needed

[JonathanREmery]

Description

I often found that the first time I would use CodeCompanion would be to add some code to the chat and ask a question. I kept running into the issue where I had not initialized the chat window yet so I would get a "No chat buffer found" warning.

I figured it would be nice if it automatically created a new chat, if one does not already exist when the user calls :CodeCompanionChat Add

Checklist

• [✓ ] I've read the CONTRIBUTING guidelines and have adhered to them in this PR
• [✓] I've updated the README and ran the make docs command

[olimorris]

And thanks for this. Such an awesome QOL improvement.