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?

[samvelyan]

Hi @BartekCupial. Many thanks for the PR. This looks great. Just have a few small points:

1. In sokoban.py, could you please rename the version of the environments from -v0 to -v1, given considerable change to their dynamics.

2. It would be wonderful to also update the documentation to reflect these changes.

[BartekCupial]

@samvelyan tell me if there sth more to be done

[samvelyan]

LGTM