fix(packets): deserialize strings starting with "-" correctly

fix(packets): use skill vnum instead of id to avoid confustion between cast id and skill vnum

feat(packets)!: let the user add the default packet converters

fix(packets): make correct fields nullable

feat(serializer): fix null representation as stated in #18

fix(packets): make cmode morph skin allow higher values

chore: remove debug stuff from source generator

feat: split packets definitions into separate assembly