feat: add dynamic score multiplier based on game level

[Erewhon-proj]

Score multiplier feature:

• Added get_score_multiplier method to DifficultyManager, which calculates a score multiplier based on the current level (10% increase per level, capped at 1.5). (src/difficulty.py)
• Introduced new settings: MAX_SCORE_MULTIPLIER and SCORE_MULTIPLIER_PER_LEVEL to control the multiplier limits and increments. (src/settings.py)

Score calculation integration:

• Updated PacMan class to hold a reference to the DifficultyManager and use the score multiplier when awarding points for pellets and frightened ghost collisions. (src/pacman.py) [1] [2] [3] [4] [5]
• Modified game loop to set and update the DifficultyManager reference in PacMan whenever the level changes. (src/main.py) [1] [2]