Add workspace context for language features in copilot chat code blocks

[dibarbet]

See microsoft/vscode-dotnettools#1361 (comment)

Currently, requests for copilot chat code blocks are made via LSP using a vscode-chat-code-block URI. Currently these files are all added to the LSP misc workspace since we don't know where they came from.

This means that language features are unable to reference context from the workspace (for example GTD on a symbol defined in a different workspace file will fail).

We should likely add a special copilot chat workspace that has references to the loaded solution from the workspace so that langauge features can reference code in the workspace.

Some open questions

1. What parts of the solution / project should be referenced by the copilot chat workspace? Just the currently opened project based on the active file in the editor? The entire solution? Or, like the interactive window in VS, we could have a command to add projects to the context.