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

FOLLOW UP

We could add a setting to let user define their own prompt #25 (comment)

[jtpio]

Should this function take a single option, instead of raw parameters like provider_name?

Wondering if this could make it easier to extend later.

[jtpio]

We could add a setting to let user define their own prompt #25 (comment)

Wondering why we would need a getter / setter on the completer for the prompt, as it is currently set to COMPLETION_SYSTEM_PROMPT by default for now. But this would likely be useful when allowing users to set their own prompts?

[brichet]

We could add a setting to let user define their own prompt #25 (comment)

Wondering why we would need a getter / setter on the completer for the prompt, as it is currently set to COMPLETION_SYSTEM_PROMPT by default for now. But this would likely be useful when allowing users to set their own prompts?

Yes, and even with the settings it could only be a writable property, since there is nothing more in the getter / setter.

[jtpio]

Thanks!