Update fields.lua

Fix:
  As it was, fields.lua was generating  a runtime error when trying to build/parse incoming 4B chunks.

Suggested changes:
- in outgoing 0x4D, renaming "Manipulation Type" to just "Type" to be consistent with incoming 0x4B's "Type"

- In incoming 0x4B: some of the comments are out of sequence.
- in incoming 0x4B, _unknown5 is listed twice. Renamed to 6 and renumbered all the following unknowns.
- in incoming 0x4B's function:  only processing the "FULL" 88-byte packet when Type matches the hardcoded  Set does not seem like the right thing to do, as there are other Type numbers with an 88-type packet. Plus there might be more. Changed function to process it differently depending on length of data instead. 
- in incoming 0x4B, renamed "Sender Name" to "Player Name" as the field is used for both for senders and recipients (depending on whether this is for inbox or outbox)

Other issues:
- Some of the existing comments with observed values are not accurate (or no longer accurate)
- 'Packet number' field does not seem to be a packet number.  unknown4 seems to be for a packet number for when they come in pairs.

There's more to do, but I wanted to push this as is for now.