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

[augustoproiete]

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.