add mixbox for csharp

csharp/examples/HelloMixbox.cs

@@ -0,0 +1,16 @@
+using System.Drawing;
+using Scrtwpns.Mixbox;
+
+public class HelloMixbox
+{
+    public static void Main(string[] args)
+    {
+        Color color1 = Color.FromArgb(0, 33, 133);  // blue
+        Color color2 = Color.FromArgb(252, 211, 0); // yellow
+        float t = 0.5f;                             // mixing ratio
+
+        Color colorMix = Color.FromArgb(Mixbox.Lerp(color1.ToArgb(), color2.ToArgb(), t));
+
+        System.Console.WriteLine(colorMix);
+    }
+}

csharp/examples/HelloMixbox.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Mixbox" Version="2.0.0" />
+  </ItemGroup>
+
+</Project>