Overwrite default prompts

[jhgoebbert]

Define better prompts for certain models (eg. Starcoder2) instead of using the default ones.

For this classmethods of BaseProvider must be overwritten:

• def update_prompt_template(self, format: str, template: str):
• def get_prompt_template(self, format) -> PromptTemplate:
• def get_chat_prompt_template(self) -> PromptTemplate:
• def get_completion_prompt_template(self) -> PromptTemplate:

Details here: https://github.com/jupyterlab/jupyter-ai/blob/v2.20.0/packages/jupyter-ai-magics/jupyter_ai_magics/providers.py#L366

Example:
TogetherAIProvider(BaseProvider, Together) -> def get_prompt_template(self, format) -> PromptTemplate:
https://github.com/jupyterlab/jupyter-ai/blob/v2.20.0/packages/jupyter-ai-magics/jupyter_ai_magics/providers.py#L736

[jhgoebbert]

For the model starcoder the tool twinny uses the following FIM (Fill-in-the-Middle) prompt generator.

getPrompt(..) -> getFimPrompt(..) -> getFimTemplateChosen(..) -> getFimPromptTemplateOther(..) ->
return <fim_prefix>${fileContext}\n${heading}${prefix}<fim_suffix>${suffix}<fim_middle>

This FIM-formatting seems also be used by Google in a slightly different way (<|fim_prefix|> -> <|fim_prefix|>): https://ai.google.dev/gemma/docs/formatting#formatting-fim
We can expect the model to answer as described here: https://ai.google.dev/gemma/docs/formatting#fim-example

More on FIM for jupyter-ai : jupyterlab/jupyter-ai#950