manage FSharp.Core version using Paket

The purpose of this is to switch to use the FSharp.Core published to NuGet.
https://github.com/fsharp/fsharp/issues/352

Unfortunately, Paket causes a bunch more noise in the diff.
No worries.

3.11.2 comes without the explicit dependency. I will add 3.11.3 to check if FAKE still builds itself