Add props file that allows us to easily isolate to a single TFM

[PureWeen]

Description of Change

Ideally we could use solution filters or something more baked in but that's not supported
dotnet/msbuild#6325

If the mechanism I have here looks like a good way to go for now then I'll add some magic to the cake script that can just write out to the props file

dotnet cake --target=VS --android --ios
dotnet cake --target=VS --tizen
dotnet cake --target=VS --catalyst
dotnet cake --target=VS --maccatalyst
dotnet cake --target=VS --windows

Motivation

Significantly improves build times if you only care about a given platform. Especially on less powerful machines