Add typed FixtureImage parameter

[e-larsson]

Not sure if I'm misunderstanding the problem but this adds the requested typehint and I don't see any circular import errors.

• Renamed image_grayscale.py -> grayscale_detection.py to make a clearer distinction from grayscale.py
• Renamed get_grayscale in grayscale_detection -> detect_grayscale to avoid confusion with grayscale.get_grayscale. I think this name is accurate for the function but please correct me if I'm wrong
• Rename detect_grayscale -> find_grayscale_locations. Same here as above, please correct me if I'm misunderstanding what the function does

Resolves #118

[local-minimum]

The renaming sounds good, but could you update branch with main before review in case that triggers the circular import?