Add option to chat without workspace context

[EwenDC]

Having a back-and-forth with Copilot is very frustrating due to the lengthly process to collect workspace information (exacerbated by VSCode forcing the "@workspace" context, and the trigger-happiness of the algorithm that determines what to collect). This creates a ~5 second delay before each response that slows down conversation unnecessarily when asking general questions not related to your codebase.

On top of that, it makes Copilot dumber as it unnecessarily prioritises the data it gets from the workspace context over the current chat context. As an example, I was asking Copilot to write a comparator function for sorting arrays in JavaScript. When it responded with a function that only sorted numbers, I asked it to rewrite the function to cover all data types. Rather than pay attention to our chat context, it assumed I must be talking about a random function in a file I had open and started trying to rewrite it. After I restated the prompt about creating a new sorting function, it just parroted back the sorting function in our codebase that I was trying to compare against a Copilot generated implementation.