refactor: moved extensions under numerics dir

Author: mrdav30

…hSharp/Extensions/Fixed3x3.Extensions.cs …merics/Extensions/Fixed3x3.Extensions.cssrc/FixedMathSharp/Extensions/Fixed3x3.Extensions.cs renamed to src/FixedMathSharp/Numerics/Extensions/Fixed3x3.Extensions.cs src/FixedMathSharp/Extensions/Fixed3x3.Extensions.cs renamed to src/FixedMathSharp/Numerics/Extensions/Fixed3x3.Extensions.cs

@@ -8,7 +8,6 @@
 #if NET8_0_OR_GREATER
 using System.Text.Json;
 using System.Text.Json.Serialization;
-
 #endif
 
 using Xunit;