Fix Beat Up's battle script to avoid an out-of-bounds array access

[sphericalice]

Description

Instead of using gBattleCommunication[0] to keep track of the current Beat Up slot for its damage calc, this PR introduces a dedicated field in gBattleStruct, beatUpSlot, which gets initialised to 0 at the start of battle. This fixes #2540, as the damage calculation was being called by AI calculations before the script for Beat Up was ran to set gBattleCommunication[0], leading to an out-of-bounds array access. For an abundance of caution, the incrementing of gBattleStruct->beatUpSlot has also been moved to occur after the damage calculation takes place, so that no shenanigans with - 1 needs to take place. I also overloaded the Cmd_trybeatup battle script command so that it's more clear that Beat Up's effect changes depending on the B_BEAT_UP setting, as before it wasn't clear by looking at that function alone that it was effectively unused when B_BEAT_UP was >= GEN_5.

Discord contact info

Spherical Ice#4683