Reduce the size of message history

[brichet]

Problem

When sending a message in the chat, the whole history is included in the message list for context. This can lead to a huge number of messages.

Proposed Solution

We should probably add a limit to the number of messages, that could be configurable with a setting.
A default value could be the last 2 messages send by the user and their responses.

Additional context

For comparison, in jupyter-ai the default value is 2 https://github.com/jupyterlab/jupyter-ai/blob/7b4586ef2adc560b457731ae07be87e348a8769e/packages/jupyter-ai/jupyter_ai/extension.py#L211

Initial discussion #41 (comment)