Existing target generic mapping

[mu-dawood]

        private partial TTarget CreateEntity<TSource, TTarget>(TSource source);  /// working
        private partial void ModifyEntity<TSource, TTarget>(TSource source, TTarget target); /// not working

Environment (please complete the following information):

• Mapperly Version: 3.2.0
• .NET Version: 7.0.310
• Target Framework: net7.0
• IDE: vs code & vs 2022
• OS: macos 14

[latonz]

Currently existing targets are not supported for generic mappings. Would be happy to accept a PR.