Add NRT annotations.

[ElektroKill]

Since .NET 6.0 projects by default have NRT(Nullable Reference Types) enabled. dnlib is not yet annotated with the NRT annotations which makes can be a big drawback when it comes to .NET 6.0 development. Adding NRT annotations in dnlib would improve the experience when working with it in .NET 6.0

[wtfsck]

I haven't done it yet because null values are used if there's some invalid metadata. If we instead return non-null values, NRT could be enabled. Probably in v4.0, that's probably a breaking change (to not return null).