Fix dead players doing damage

[Nico-Sanchez]

Motivation

Players were doing damage even after death.
Closes #551

Summary of changes

• Execute mechanics from player if player is alive only
• Make execute_mechanic/4 private

How to test it?

Play a game and execute skills when about to die. Everything but player mechanics (proyectiles, pools) should still work, but not the players' mechanics (circle_hit, etc).

Here's a video with the fix:

Uploading Screen Recording 2024-12-12 at 6.41.51 PM.mov…

Checklist

• Tested the changes locally.
• Reviewed the changes on GitHub, line by line.
• This change requires new documentation.
  • Documentation has been added/updated.