create boss levels

[azhou202]

at some levels (ex: 5, 10) have a boss show up

[azhou202]

boss features

• larger size (close to 70% of screen)
• larger health (takes multiple bullets to kill)
• trigger a boss health bar or numeric counter
• slower movement speed
• fire wide beam laser
• be able to fire lasers
• base amount of lasers active is 3
• not move down the screen
• only way to move past the boss is kill it, loss means getting shot by boss
• increases points by 10x normal alien
• as levels progress, bosses gain more health and can fire more lasers