Most WinForms/WPF users (many from big enterprises) are still fine with Visual Studio for Windows (the .NET Core based WinForms/WPF designers are much better), so there isn't enough push to add such support to VS Code by Microsoft. So, I don't think there is any hope for alternatives (Rider is not free and also a monster of its own).
If you are eager to get something in this field, you can write your own VS Code extension to wrap over SharpDevelop's legacy, but likely you are just a few who need that.
Update: I have made one called vscode-wpf, language server, Hot Reload, and visual designer are all integrated.
