Organize C# binding example into separate Regorus nuget package and
a test app.
The nuget package targets netstandard 2.0 and 2.1.
The test app is tested for netframework 8.0.
Implement IDisposable for Engine cleanup.
Also remove net40 example. Can be added back later if needed.
Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>
