Nullable reference types

[MrJul]

This PR enables nullable reference types in all XamlX projects.

Most Avalonia projects now use nullable annotations, but some core ones such as Markup.Xaml.Loader can't because XamlX isn't nullable aware. This PR fixes that.

Notable changes:

• A couple classes with non-nullable members and a default constructor now have a constructor with parameters instead.
• DeclaringType is now non-null and exposed on IXamlMember directly.
• GetMethod overloads matching the FindMethod ones have been added and used in place of FindMethod, as most places in emit code expected methods to actually be found.
• TypeReferenceResolver.ResolveType returns a non null type. There were some code paths accepting null, but at the end of the day ResolveTypeCore was called which throws if the type couldn't be resolved, which callers expected.
• CecilType.GetEnumUnderlyingType and CecilField.GetLiteralValue now throw instead of returning null for non-enum/literal. This matches the SRE implementation.