feat: spawn enemies when they became near the player

[erickzanardo]

Description

Changes the game so the enemies are only spawned when the player gets to a certain proximity of them, and when the enemies gets far from the player, they are automatically removed.

This was necessary in order to keep the game perfomance, no matter how many enemies are in the level.

Note: Needs to wait #16 to land before merging this.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore