Implement Image.split() and Image.getbands()

[bgunebakan]

What is solved?

Implements Image.split() and Image.getbands() on the Image class, closing #14. Both methods are Pillow-compatible and cover all currently supported image modes (L, LA, RGB, RGBA). Also fixes a gap in Image.new(): it now correctly accepts a 2-tuple (luma, alpha) color value when creating LA-mode images.

Changes

• Image.split() splits an image into its individual channels and returns a tuple[Image, ...] of independent L-mode (grayscale) copies, one per band. Splitting a single-band L image returns a 1-tuple copy.
• Image.getbands() returns a tuple[str, ...] of band name strings for the image's mode (e.g. ('R', 'G', 'B') for RGB).

Bug fix

• Image.new() now accepts a (luma, alpha) 2-tuple color for LA-mode images (color parsing extended in utils.rs).

Checklist

• Connected to Issue implement Image.split() and Image.getbands() #14
• Tests coverage increased for new features
• API docs updated
• Changelog updated
• Pillow compatibility table updated