Add type hinting for Core API

[arjun-kg]

Description

• Add type hinting for everything under minigrid/core.
• Also minor refactoring, removing duplicate code, simplifying complex statements etc.

I had some trouble with pyright, there was a particular case which I wasn't sure how to deal with,

suppose,

def func(x: Optional[int] = None):
   if x is None:
      x = 3
   ... 
   y = x + 5 # pyright has a problem with this saying None type does not support addition 

My solution was to add a assert x is not None above where the problem occurs to fix this, I might be missing a simpler fix.

Type of change

• [x] New feature (non-breaking change which adds functionality)

Checklist:

• [x] I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• [ ] 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
• [ ] 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