Add primary color dithering to ImageOps

[Marcelo-MConti]

What does this PR do?

This PR adds a new image operation to PIL.ImageOps that applies a
primary-color dithering filter (RGB only).

The filter:

• Thresholds RGB channels to primary colors (0 or 255)
• Applies a simple dithering strategy to preserve local contrast
• Returns a new image, leaving the input unchanged

Why is this useful?

Primary-color dithering can be useful for:

• Educational purposes (image processing fundamentals)
• Artistic and stylized image transformations
• Low-color or hardware-constrained displays

What is included?

• New function: ImageOps.dither_primary
• Unit tests covering basic behavior and output properties
• Documentation added to the ImageOps module

Notes

• The function expects an RGB image
• The implementation follows existing ImageOps conventions

Before Filter

After Filter

[radarhere]

I've created Marcelo-MConti#1 to fix the lint failures.

[radarhere]

I've created Marcelo-MConti#2 with an idea - process the image by band, instead of all bands at once. I find it is a bit faster, and neater.