Add check for fields not existing when writing container

[u9g]

There is a common problem that when the protocol gets updated some fields are silently writing garbage, so let's try to catch that.

Compiled container writes throw Missing field 'x' when a field is undefined and its type is one of protodef's value types: the numeric types, varint, bool, pstring, cstring, buffer, bitfield, mapper, array, count, container, or an alias of one. Bitfield members are always required.

Fields typed void, switch, option, or a type protodef does not define are not checked: those can encode an absent value (a switch whose default is void, prismarine-nbt's optional tags), and minecraft-protocol sends them undefined on purpose (use_entity with mouse: 0, a Slot without NBT on 1.13-1.20.4).

The interpreter is unchanged: it does not keep the JSON of a named type, so it cannot tell a switch alias from a varint alias.

minecraft-protocol's packetTest seeds SpawnInfo without seaLevel (1.21.2+); that varint was written as 0 before and now fails the login/respawn cases with Missing field 'seaLevel'.

[extremeheat]

Interpreter should probably be updated also