Feature Request: Add folding to the extension

It appears that #8 had a very similar request to this, but at the time, VSCode didn't really allow folding. They have since [added language aware folding](https://code.visualstudio.com/updates/v1_23#_folding-provider-api), and it would be beneficial to include the proper data to allow folding.

I suspect whatever is done to allow folding in VSCode could also be used in the [Visual Studio Extension](https://github.com/cake-build/cake-vs) as well.

Start by reviewing the VSCode language-aware folding provider API and the existing Cake VSCode extension structure. Determine what folding data the extension can provide; the work is done when the extension enables appropriate folding in VSCode, with any relevant approach also considered for the linked Visual Studio Extension.

Written by the indexing model from the issue text.