Difficulty Increase

There should be a `DifficultyManager` which will set various parameters within the `EnemySpawner` object and also keep track of the difficulty we are on.

Should have a state machine for each level which will set various combinations of enemies to be spawned in.

One possible implementation:
Level 1:
- Basic Waves, low health
Level 2:
- Basic Waves, low health
- Tank Waves, high health
Level 3:
- Level 2
- 1 sideswiper spawns in every 5 seconds
Level 4:
- Level 3, but 2 sideswipers spawn in every 5 seconds

Start by locating the EnemySpawner object and reviewing how enemy waves, health, and spawn timing are currently configured. Then determine where a DifficultyManager and per-level state machine would fit, using the listed level combinations as the acceptance criteria for the difficulty progression.

Written by the indexing model from the issue text.