Add icons metadata files

[blakegearin]

Summary

• Added build script for generating a metadata directory with JSON files for icons and categories

Notes

• This treats the docs/content/icons/*.md files as the source of truth

• Pulling in js-yaml seemed to be the best way to convert Frontmatter to JSON; there are other similar packages but they're significantly less popular and battle-tested

• If YML is preferred over JSON, please let me know and I can adapt the script

• This is common to provide among icon libraries since it enables search (Font Awesome, Ionicons, Lucide, Tabler)

• The primary concern this is trying to address is that tags & categories are not available in bulk like icons are with the sprite

  • Related issue: #1384

[blakegearin]

Rebased to fix conflicts.

@XhmikosR Let me know if you have any feedback.