Add separate exp modifier for BBM enemies

[13masj]

Separate exp modifiers for enemies in and outside of Bitterblack Maze game mode.

Tested settings below:
EnemyExpModifer: 2.0
BBMEnemyExpModifer: 1.0
goblin (lv1): 20 exp
Netherworld Familiar (lv8, BBM): 175 exp
A Man Eating Ogre (enemy): 15200 exp
A Man Eating Ogre (Quest): 1020 exp

EnemyExpModifer: 1.0
BBMEnemyExpModifer: 2.0
goblin (lv1): 10 exp
Netherworld Familiar (lv8, BBM): 350 exp
A Man Eating Ogre (enemy): 7600 exp
A Man Eating Ogre (Quest): 1020 exp

Checklist:

• The project compiles
• The PR targets develop branch