Only show Avalonia project items in Avalonia projects.

[stevemonaco]

Is your feature request related to a problem? Please describe.

When using "Add New Item..." in Visual Studio, Avalonia-related items (eg. Avalonia UserControl) are always shown in the dialog for all C# projects, even if the user is trying to add something to a Console app. Other project templates, such as WPF and WinForms, do not do this.

Rider somehow already does this. It also shows Avalonia-related items in the Add context menu (VS doesn't, only shows in the dialog).

Describe the solution you'd like

The Avalonia NuGet package adds a Project Capability called "Avalonia". If we add a constraint to the templates according to https://github.com/dotnet/templating/wiki/Constraints#project-capabilities then maybe VS will respect it. Untested at the moment. VS seems to respect the "tags" for "language" in template.json at least.

Describe alternatives you've considered

No response

Additional context

I can try to test + PR this but might take quite some time as I haven't built this repo or worked with .NET templating before.

From a community complaint.