feat(packets): add nullable wrapper to represent a compoud type that may be null as a whole

feat(packets): add PerparedLevel to PacketStringEnumerator's levels to allow multiple prepared levels for nested packets

chore: update dependencies

chore: update to .NET 7

feat: make projects available for netstandard 2.1

chore: add repository url and license to nuget

chore: add nuget description

feat: split packets definitions into separate assembly