Add initial system prompt in ChatHandler and completion

[brichet]

Fixes #25

Use the same prompts as jupyter-ai (except for the model name, which is not part of the BaseChatModel and may not be provided by all chat model).

NOTES

1. Currently the completion prompt is not used by the codestral completer, because the codestral completer uses a text completion model, which doesn't seem to allow such prompts.

2. For a follow up PR, we should:

   • use a chat model for codestral instead of LLM (as expressed in Improve the handling of AI clients and API keys #9 (comment))

     do we need to instantiate both a MistralAI and ChatMistralAI clients?

   • create a BaseCompleter class that may fit with most of the completers