Added "Easy" mod.
- Halves all difficulty values and grants 3 "lives" instead of 1, with a score multiplier of 0.5x.

Other changes:
- Fixed score display in game state if score exceeds 8 digits.
- Added a "HP_DRAIN_MULTIPLIER" constant for steady HP drain/increase (to replace numeric constants).

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>