Enable IntelliSense without having to open a folder in VS Code (i.e. single file too)

Currently, in order for IntelliSense to work, a user must open a folder in VS Code, which is not always what's desired in some common day-to-day scenarios:

1.) It's easy to drag & drop a `build.cake` from Windows Explorer when the user wants to make a simple change to that one file

2.) It's easy to type `code build.cake` from a command-prompt/powershell/bash when the user wants to make a simple change to that one file

In both scenarios above the user is required to open the entire folder, with tens or hundreds of files which are all analyzed by the `N` VS Code extensions that the user has, including OmniSharp, which makes the whole workflow much slower than it could be if we implement this improvement.

Start by reviewing the Cake VS Code extension's IntelliSense entry points and how they detect an opened folder. Exercise both single-file workflows described in the issue, using drag-and-drop and `code build.cake`; done means IntelliSense works without opening the containing folder while preserving the existing folder workflow.

Written by the indexing model from the issue text.