Added Net5 target framework

+docs

small improvements to MeshBuilder.AddMesh

Improved matrix validation.

Updated test files to point to the new BabylonJS assets.

last minute fixes

Renamed Schema2Toolkit to just Toolkit to remove a naming redundancy;…

… this is a small breaking change.

Improved performance when validating large scene node trees.

Code Cleanup.

API Improvements to AffineTransform.cs

Added AffineTransform multiply based on code from: https://gist.github.com/ptasev/bce22ca61d9da83a702c2011229b76ab

updated version

updated nuget packages

updated preview builder

Improved SceneBuilder.

Updated Nugets.