Add Files in Frameworks as Context

[owenzhao]

Currently Copilot can only use files that I created in my project as context. However, sometimes AI can't use some framework properly. For example, AI may keeps using some properties relate to a framework with the wrong types. Currently, the only way to solve this kind of issue is to copy the full code of the source file and paste it in the prompt window. However, it will be much better to choose the file of the framework as context.