Add UnreachableException, which was introduced in .NET 7 (#78)

* Add `UnreachableException`, which was introduced in .NET 7

* Added test to check targetframework compatiblity

src/Polyfill/UnreachableException.cs

@@ -0,0 +1,57 @@
+// <auto-generated />
+
+#if !NET7_0_OR_GREATER
+
+#nullable enable
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
+using System.Text;
+
+namespace System.Diagnostics;
+
+/// <summary>
+/// Exception thrown when the program executes an instruction that was thought to be unreachable.
+/// </summary>
+/// <seealso href="https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.unreachableexception"/>
+/// <seealso href="https://source.dot.net/#System.Private.CoreLib/src/libraries/System.Private.CoreLib/src/System/Diagnostics/UnreachableException.cs"/>
+[ExcludeFromCodeCoverage]
+[DebuggerNonUserCode]
+#if PolyPublic
+public
+#endif
+sealed class UnreachableException : Exception
+{
+    /// <summary>
+    /// Initializes a new instance of the <see cref="UnreachableException"/> class with the default error message.
+    /// </summary>
+    public UnreachableException()
+        : base("The program executed an instruction that was thought to be unreachable.")
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="UnreachableException"/>
+    /// class with a specified error message.
+    /// </summary>
+    /// <param name="message">The error message that explains the reason for the exception.</param>
+    public UnreachableException(string? message)
+        : base(message)
+    {
+    }
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="UnreachableException"/>
+    /// class with a specified error message and a reference to the inner exception that is the cause of
+    /// this exception.
+    /// </summary>
+    /// <param name="message">The error message that explains the reason for the exception.</param>
+    /// <param name="innerException">The exception that is the cause of the current exception.</param>
+    public UnreachableException(string? message, Exception? innerException)
+        : base(message, innerException)
+    {
+    }
+}
+
+#endif

src/Tests/UnreachableExceptionTests.cs

@@ -0,0 +1,11 @@
+namespace Tests;
+
+[TestFixture]
+public class UnreachableExceptionTests
+{
+    [Test]
+    public void UnreachableException_Compatiblity_with_all_TargetFrameworks()
+    {
+        _ = Assert.Throws<UnreachableException>(() => throw new UnreachableException());
+    }
+}