Copilot chat code blocks should be able to reference code from other copilot chat blocks

[dibarbet]

From microsoft/vscode-dotnettools#1361 (comment)

It can be useful for C# code inside copilot chat code to reference previous (or subsequent) code blocks generated by copilot chat.

Currently copilot chat code block contents goes into the misc workspace with a URI, and have no references to other misc files. Likely we need to create a copilot chat workspace and have all the code block contents go into the same project. This should allow language features to see the code from other code blocks.