Optimise DefaultPlatformModeDetector on csproj sdk platforms

[glennawatson]

.NET 6 introduced some new MSBuild properties 'UseWindowsForms' 'UseMaui' and 'UseWPF'

We can take advantage of those in the DefaultPlatformModeDetector to optimise the experience for the user.

https://github.com/reactiveui/splat/blob/main/src/Splat.Drawing/DefaultPlatformModeDetector.cs

Provide #if overrides in those scenarios.