Add BabyAI levels

[saleml]

Description

Add levels from babyai to envs.babyai.

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

• [x] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[saleml]

What does the verifier do?

verifier.py defines Instructions classes, that inherit from the abstract Instr class. A simple instruction (object of type ActionInstr) contains an environment (object of type MiniGridEnv), and the description of an object (object of type ObjDesc). A sequential instruction (object of type SeqInstr) chains multiple simple instructions. These instruction objects are created automatically by LevelGen or RoomGridLevel objects, that generate the levels / environments.

Each instruction has a natural language representation, obtained via the surface method. The verifier endows each Instruction class with a verify method, that is called after every step, that checks whether the instruction has been completed or not.

Do we want tests to check it?

That doesn't seem like a straightforward task, unless we bring in the bot from babyai, which solves BabyAI levels using only information available to any learning agent (including partial observability).