Rename MON_ID to MON_TRAINER_ID

[SnorlaxMonster]

The constant name MON_ID implies it relates to the ID number of the Pokémon itself; however, it actually refers to the Trainer ID number of the Pokémon's Original Trainer. Renaming it to MON_TRAINER_ID makes it substantially more clear what its meaning is.

The original definition of MON_ID is preserved in macros/legacy.asm