feat(tree-wide)!: introduce standard userstyles library

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Proof-of-concept that sets up a "userstyles standard library" that is imported and utilized to include the palette(s), variable definitions, copy-pasted utils, etc.

To-do

• Publish lib/ folder to https://userstyles.catppuccin.com/lib
  • Set userstyles.catppuccin.com domain
  • Merge to main and deploy to primary site
  • Replace https://raw.githubusercontent.com/catppuccin/userstyles/refs/heads/feat/importable-lib/lib/lib.less URLs with perma https://userstyles.catppuccin.com/lib/lib.less
• Refactor all userstyles tree-wide to use new lib
  • Remove @catppuccin and @catppuccin-filters palettes
  • Replace palette variable decls with #userstyles.palette()
  • Replace default styles with #userstyles.defaults()
  • Replace palette CSS variable decls with #userstyles.css-variables()
  • Replace #rgbify and #hslify uses
  • Replace syntax highlighting import URLs with lib functions? (Would allow us to update all usages of the syntax highlighting libraries to a new version at once, for better or worse). #userstyles.highlighting.prismjs("3.1") (with version param?)
• Test effects of importing lib in every userstyle - network requests, caching.
• Test for breakage.
• Generate lib with scripts?
  • New tints-and-shades palette is script-generated, see feat!: introduce tints and shades palette #1737.
• Investigate using "lib" structure for other shared userstyles code
  • Google's M3 variables?
  • Bootstrap variables?
  • MediaWiki?