Checkpoint system

[jyee27]

Add a checkpoint system.

• When the player hits an invisible trigger, activate the checkpoint. The player starts at the checkpoint after dying.
• Add a respawn point visual. A flag that goes up when activated is fine but something scifi would be better.
• Persist between scene restarts:
  • Checkpoint
  • Scrap collected
  • Other map states e.g. shield generator

Other considerations:

• We don't want players falling into space and getting frustrated at how long it takes to die.
• Adjust the game balance so it's difficult, but dying doesn't set you back super far.