feat(plugin): map iconSets for string prefixes

[gapurov]

• If you list prefixes as plain strings, we now check iconSets first instead of always assuming @iconify-json/.
• Existing source-based configs keep working as-is.
• Added a quick test to cover local icon sets with clean selectors.

Why

With CSS-only configs, clean selectors were failing for local icon sets because the plugin always tried to load @iconify-json/<prefix>. This makes icon-sets: from-folder(...) work the way you’d expect.

This is now possible:

 @plugin "@iconify/tailwind4" {
    icon-sets: from-folder(custom-icon-prefix, "./src/assets/icons/custom");
    prefixes: custom-icon-prefix, mdi-light;
 }

[gapurov]

@cyberalien, friendly asking to review this small PR, and to check whether it aligns with your vision. Thank you.

[cyberalien]

Sorry for delay. At first glance it looks good, but I'll need to test it. It might take few days.