Consider supporting pre-baked .NET's in extension

VSCode extensions today cannot have different flavors for different platforms; however, this is changing with: https://github.com/microsoft/vscode/issues/23251#issuecomment-458075818

We can take a model where we have a default .NET included in the box to avoid dynamic acquisition; and then if another extension attempts to download a different version of .NET not included in the box we can fall back.