Change Environment.GetFolderPath to be compatible with tvOS

[marek-safar]

As part of the mobile workloads migration, we need to ensure the apps are behaving consistently when calling into Environment.GetFolderPath. There is already some implementation for iOS but it looks like it's not fully compatible with existing Mono code

The second part is to ensure there is full code coverage for all SpecialFolder values.

[marek-safar]

/cc @EgorBo

[akoeplinger]

We still have an important TODO that needs to be fixed for tvOS:

runtime/src/mono/netcore/System.Private.CoreLib/src/System/Environment.iOS.cs

Lines 39 to 41 in f48c8d5

	// TODO: fix for tvOS (https://github.com/dotnet/runtime/issues/34007)
	// The "normal" NSDocumentDirectory is a read-only directory on tvOS
	// and that breaks a lot of assumptions in the runtime and the BCL

Reopening to track that.

[marek-safar]

@steveisok please assign