[NUI] VectorGraphics: Move enums out of class

[NUI] VectorGraphics: Move enums out of class

Move enums to VectorGraphicsConstants.cs to avoid nested types.
And add a warning message to FillRuleType, StrokeCapType,
and StrokeJoinType enums of the Shape class that have become ACR.
https://docs.microsoft.com/en-us/dotnet/standard/design-guidelines/nested-types