Format/size detection performance improvements

[b0o]

Adds fast, lua-based format detection and dimension reading for common image formats, falling back to ImageMagick when needed. This avoids spawning external processes for basic operations, making things noticeably snappier, especially when using image.nvim's API from other plugins (my use-case).

• Extended built-in format detection using magic numbers instead of relying on ImageMagick
• Direct header parsing for image dimensions (PNG, JPEG, WebP, GIF, BMP, ICO, HEIC, AVIF)
• Added test suite with sample images and edge cases
• Set up basic GitHub Actions CI for running tests

The lua implementations are significantly faster but slightly less forgiving with malformed files compared to ImageMagick. We fall back to the existing methods when lua-based parsing fails, so everything should be backward compatible. More test images might be beneficial, especially for heic/avif formats.

Benchmarking the time to run from_image:

• With magick_rock:
  • For a 2880 x 1800 PNG file:
    • Before: 283.36ms
    • After: 0.50ms
  • For a 208 x 209 PNG file:
    • Before: 4.96ms
    • After: 0.36ms
• With magick_cli:
  • For a 2880 x 1800 PNG file:
    • Before: 294.32ms
    • After: 0.48ms
  • For a 208 x 209 PNG file:
    • Before: 32.82ms
    • After: 0.16ms

Similar speedups for other formats.

Note: most of the logic in dimensions.lua was generated with Anthropic's Claude.

[3rd]

Absolutely amazing effort, thank you so much on behalf of everyone!

[3rd]

Hey @b0o, been having some issues with the tests, have you seen this before? https://github.com/3rd/image.nvim/actions/runs/12633430737/job/35199022999
It's weird that I can't run them on local NixOS either, with working luarocks, busted, etc.
I'm trying hard to like luarocks but as a whole package it's a pretty clunky experience, might replace busted with something else and possibly rip out as many rock-based deps as possible.