Make grenades damage characters

## Problem
Grenades don't do damage.

## Proposal
Make them do damage in an area. Let's start with uniform damage in the whole area.

## Alternatives
Fall-off damage from the center of the explosion (harder to code, we can do it in a later issue).

## Minimum added value
- Grenade has an area of effect for the explosion
- This area detects a body. If the body is an Alpaco, damage it. Use [groups](https://docs.godotengine.org/en/stable/tutorials/scripting/groups.html) (make the player part of the group "alpacos" and check the body with the function is_in_group.

## Additional context
Add any other context or screenshots about the feature request here.

Start by locating the grenade explosion behavior and the player character's damage handling in the Godot project. Add the stated area-of-effect behavior so bodies identified as Alpacos receive uniform damage, then verify that the explosion detects Alpacos and damages them.

Written by the indexing model from the issue text.