Add PIL fallback for image resizing

[vikhyat]

This PR adds a fallback to PIL/Pillow for image resizing when PyVips is not available. Changes include:

• Modified image_crops.py to use PIL when PyVips import fails
• Added comprehensive tests to verify both PyVips and PIL implementations produce similar results
• Test results show max pixel difference of 2.0 and mean difference of 0.27 between implementations

This change makes the library more accessible to users who may not have PyVips installed.