Implement @embedNullable for serialization

[WebFreak001]

Fixes #1541 and is an alternative, simpler implementation of #1650

this adds an embedNullable UDA which specially treats std.typecons.Nullable without values as not defined. This makes it ignored during serialization.

This transparently makes the type hidden for a serializer if it's not given and thus allows special states like:

• undefined == Nullable!T.init (not in output at all)
• null == Nullable!T(null) (in output like usual for T == null)
• value == Nullable!T(value) (in output like usual)

Use case:

• patch structs where only the fields which should be applied are set to non-nullified values, so they allow patching to null and any other value

Doesn't repurpose @optional to keep backwards compatibility and avoid ambiguous definitions of "what's the ignored value" for an optional value.

[s-ludwig]

I'll still have to look at the code in detail, but the approach looks good to me! Can you do a rebase to (hopefully) make the CI pass?

[WebFreak001]

ok done

[s-ludwig]

It might make sense to put some additional thought into alternative names for the attribute in the future, but there was no name that immediately crossed my mind, so in any case, let's go with this.