New MonoGame.Framework Nuget package

[tomspilman]

Intent

We want to create a new singular MonoGame.Framework Nuget that to begin with contains the new native framework and supports the new DesktopVK and WindowsDX12 backends.

This Nuget would be a preview and is still experimental, but eventually may supersede all other MonoGame.Framework.XXX Nugets.

Motivation

The intent is to make it easier for developers to start using these new platforms by having a singular NuGet that supports them.

[ThomasFOG]

Postponed until at least DesktopVK is shippable.