feat: add shepard method backend

[ashish0kumar]

This PR introduces Shepard's inverse distance weighting method as a new color correction backend, providing smooth and high-quality color interpolation for theme conversion.

Usage

# Use Shepard's method with default settings
gowall convert input.jpg --theme catppuccin --backend shepard

# Customize parameters
gowall convert input.jpg --theme nord --backend shepard --nearest 20 --power 3.0

# Available backends for comparison
gowall convert input.jpg --theme catppuccin --backend nn     	# Nearest neighbor
gowall convert input.jpg --theme catppuccin --backend rbf    	# RBF (default)
gowall convert input.jpg --theme catppuccin --backend shepard 	# Shepard's method

Background

This PR was created following your suggestion on my Reddit post about the tint project on r/unixporn, where you mentioned it would be a great fit to add Shepard's method as a backend to gowall instead of maintaining separate tools

[Achno]

Hello @ashish0kumar ,

thanks for opening the PR, just giving you some heads up that it's exam season currently and i won't be able to review your PR until 2.5 weeks from now. Sorry for the inconvenience, but i don't have any time to spare, i hope you understand.

[ashish0kumar]

Hello @ashish0kumar ,

thanks for opening the PR, just giving you some heads up that it's exam season currently and i won't be able to review your PR until 2.5 weeks from now. Sorry for the inconvenience, but i don't have any time to spare, i hope you understand.

No worries at all, completely understand, and really appreciate the heads-up!
Wishing you all the best for your exams!