Improve how VS Code IL Viewer locates the correct file

[josephwoodward]

See snippet from this issue raise

What's happening is, because VS Code is a text editor it has no concept of a type (just a file), VS Code IL Viewer sends the filename to the background service which locates the type by the filename. I'm going to have to look to see if I can load the file, and parse the type, or use OmniSharp to get the type.

The difficulty/question comes, when there's more than one type in a file - which one do we pick?