Global Prompt for Copilot and Project Level Prompt

[owenzhao]

The current Copilot lacks the capability for system and project-level prompting. For example, we could set preferences for certain features, such as requiring Copilot to write code with more detailed comments, or specifying that APIs should be called using iOS 17 or higher. Since these needs are recurring, having an overall prompt would eliminate the need to input them anew each time.