Add llm-mistral plugin

[danielcorin]

Created for blog post.
Nix-managed llm with llm-mistral plugin added

❯ llm models
OpenAI Chat: gpt-3.5-turbo (aliases: 3.5, chatgpt)
OpenAI Chat: gpt-3.5-turbo-16k (aliases: chatgpt-16k, 3.5-16k)
OpenAI Chat: gpt-4 (aliases: 4, gpt4)
OpenAI Chat: gpt-4-32k (aliases: 4-32k)
OpenAI Chat: gpt-4-1106-preview
OpenAI Chat: gpt-4-0125-preview
OpenAI Chat: gpt-4-turbo-preview (aliases: gpt-4-turbo, 4-turbo, 4t)
OpenAI Completion: gpt-3.5-turbo-instruct (aliases: 3.5-instruct, chatgpt-instruct)
Mistral: mistral/open-mistral-7b
Mistral: mistral/mistral-tiny-2312
Mistral: mistral/mistral-tiny (aliases: mistral-tiny)
Mistral: mistral/open-mixtral-8x7b
Mistral: mistral/mistral-small-2312
Mistral: mistral/mistral-small (aliases: mistral-small)
Mistral: mistral/mistral-small-2402
Mistral: mistral/mistral-small-latest
Mistral: mistral/mistral-medium-latest
Mistral: mistral/mistral-medium-2312
Mistral: mistral/mistral-medium (aliases: mistral-medium)
Mistral: mistral/mistral-large-latest (aliases: mistral-large)
Mistral: mistral/mistral-large-2402