[Icons] Add support for suffixes

[seb-jean]

I use the Heroicons icon set, which is divided into four groups of icons that Iconify calls suffixes:

• Outline 24x24
• Solid 24x24
• Solid 20x20
• Solid 16x16

The following endpoint displays the four suffixes:
https://api.iconify.design/collection?prefix=heroicons

I would like to set different default attributes and classes for each icon suffix, but currently this is only possible for icon sets. It would be useful to be able to configure each icon suffix individually.