feat(languages): add C# support

challenges/hello-world/solutions/cs/function/README.md

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+# hello-world/cs/function
+
+Created by [@Divlo](https://github.com/Divlo) on 24 June 2021.

challenges/hello-world/solutions/cs/function/Solution.cs

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+namespace Solution
+{
+  class Program
+  {
+    static void Main()
+    {
+      string line;
+      while ((line = System.Console.ReadLine()) != null)
+      {
+
+        System.Console.WriteLine($"Hello, {line}!");
+      }
+    }
+  }
+}

challenges/hello-world/solutions/cs/function/cs.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net5.0</TargetFramework>
+  </PropertyGroup>
+</Project>