Add support for [MemberNotNull] and [MemberNotNullWhen]

[manuelroemer]

New nullability attributes are currently being introduced (see https://github.com/dotnet/runtime/blob/master/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs).

These should be added to Nullable as soon as:

• They are available in a .NET release/are finalized and no longer prone to change.
• It is verified that the compiler also supports them when they are declared as internal attributes (otherwise adding them to this package has no benefit).

[manuelroemer]

Will be available in 1.3.0.