Fixed Sokoban crash bug

[BartekCupial]

Associated issue: #4

Description

This PR fixes a crash in the Sokoban environments when the agent character (@) disappears from observation. The crash occurs because _object_positions() returns an empty list when the agent is not found, causing an IndexError when accessing the first element

Changes Made

• fixed index error which occured in death screen agent_pos = next((pos for pos in self._object_positions(observation, "@")), None)
• added food to all maps (the same amount as in nle), because some maps are impossible to solve without first starving to death
• changed action space so we can interact with food (PICKUP, EAT)

Testing

• Verified crash is fixed with MiniHack-Sokoban1b-v0

import minihack
import gymnasium as gym

env = gym.make("MiniHack-Sokoban1b-v0")

env.reset()
env.render()
done = False

while not done:
    obs, rew, term, trun, info = env.step(env.action_space.sample())
    done = term or trun
    env.render()

Closes #4

[BartekCupial]

@samvelyan can we merge this?