Add support for multiplicative/divisive/percentage modifiers to equipment modifiers

[PaschalisPipidis]

Add support for more types of modifiers beyond simple additive and subtractive modifiers similarly to how cost modifiers work for equipment modifiers.
Example use cases:

• 0.8 * half-damage range and maximum range equipment modifier for pistols using subsonic ammunition as per HT165.
• 60% damage, half damage range and maximum range equipment modifier for PDWs and rifles using subsonic ammunition as per HT165.
• DR / 2 for equipment modifier for armors at under 1/3 of their HP as per Low-Tech Companion 2 page 25