Fixed minor bug where isFinite would not be used if the version were .NET core.

src/Shared/_Extensions.cs

@@ -46,7 +46,7 @@ namespace SharpGLTF
 
         internal static bool _IsFinite(this float value)
         {
-            #if NETSTANDARD2_1_OR_GREATER
+            #if NETSTANDARD2_1_OR_GREATER || NETCOREAPP3_1_OR_GREATER
             return float.IsFinite(value);
             #else
             return !(float.IsNaN(value) || float.IsInfinity(value));