Improve ShouldBe<T>() type assertion by having it throw MatchException.

src/Fixie.Tests/Assertions/AssertionExtensions.cs

@@ -62,7 +62,7 @@ public static T ShouldBe<T>(this object? actual)
             if (actual is T typed)
                 return typed;
 
-            throw new AssertException(typeof(T), actual?.GetType());
+            throw new MatchException(typeof(T).ToString(), actual?.GetType().ToString());
         }
 
         public static void ShouldBeEmpty<T>(this IEnumerable<T> collection)