Add support for aborting chat completion responses

[srikanth235]

This will allows users to easily abort irrelevant chat completion responses. This way they will have geater control over the conversation flow.

[trymeouteh]

Would like to see a stop button or stop generating button as well

This is also a feature request for Rubberduck, the project Privy is a fork of https://github.com/rubberduck-ai/rubberduck-vscode/issues/121