Carry context in response to user messages

[nikomatsakis]

The followUp function is providing zero context:

https://github.com/nikomatsakis/intorust.vscode/blob/ffff983688652133fe4a5fc80221fba683a6e6f5/src/advisor.ts#L25-L36

This results in amusingly bad responses. e.g., I asked how to convert a number into a string and it talked about Python.

[kwdev]

I updated this a little bit by adding a user prompt in the follow-up to keep it Rust-focused.
#17

[kwdev]

Implemented very basic context in #18
It does not yet summarize or pay attention to the size of the context growing yet.

[kwdev]

Ensure token size stays within bounds for the model - #19