fix(packets): parse null upgrade rare sub packet correctly

Merge pull request #47 from Rutherther/feat/pass-nullable

Pass CanBeNull to string serializer and converter

feat(packets): pass nullable to packet serializers

fix(packets): make UpgradeRare packet deserialize correctly

fix(packets): make UpgradeRareSubPacket converter work with "0"

chore: move PacketSerializer types to correct namespace

Merge pull request #32 from Rutherther/combat

Add combat extension

fix(packets): parse 0 UpgradeRareSubPacket as 0, 0

feat: split packets definitions into separate assembly