Add method: set_tunic_sword_animation(animation)

This method would allow developers to change the default "sword" animation of the hero's tunic to a custom animation of their choice. Additionally, the engine would logically associate the specified animation with a corresponding sword sprite.

For example:

If the method is called with the animation "sword_reverse_slash", the engine would automatically expect and use the sword sprite located at "hero/sword_reverse_slash(variant)".
This maintains the current logic where the default "sword" animation is tied to the sprite "hero/sword(variant)".

The goal is to provide more flexibility for custom weapon designs while keeping the behavior consistent with the existing framework of Solarus. This would allow developers to implement unique sword animations and their corresponding sprites without needing extensive workarounds or custom systems.