Implementation of game "Alien Invasion".

Main features:

• A player ship that can move (<-/->), shoot bullets (space), and destroy aliens.
• A fleet of aliens that can move. Their speed increases with the level of the player.
• Info board:
  • Current score
  • Highest score
  • Current level
  • Number of lives left
• The game starts by clicking the "Start" button with the mouse.
• Exit with ESQ and ❌