Add an API to get the current Uno Platform target at runtime

[MartinZikmund]

What would you like to be added:

Currently it is possible to conditionally compile code to run on specific platforms, but there is no easy way to check the current Uno target at runtime. This would be especially useful to differentiate between Skia targets.

This could be a simple static method that would return an enum with Windows, iOS, SkiaWpf, SkiaGtk, etc.

Why is this needed:

Runtime checks

For which Platform:

All

[BernhardPollerspoeck]

@MartinZikmund i hope you dont mind me highjacking your issue.
 

i am at the moment working out on where i could set the current value. Any guidance here is apprechiated. 

i plan this week to create a first draft PR for the extension of the PlatformHelper class and creation of the new enum.