Add :mini and :micro variants for Heroicons

[bslobodin]

Summary

Adds the two missing Heroicon variants — :mini (20x20, solid fill) and :micro (16x16, solid fill) — to complement the existing :solid and :outline variants.

Closes #39

Details

The Heroicons library ships 4 variants, but only 2 were supported:

Variant	Size	Style	Status
outline	24x24	1.5px stroke	Existing
solid	24x24	Solid fill	Existing
mini	20x20	Solid fill	New
micro	16x16	Solid fill	New

8 icons that don't have a micro variant upstream (arrow-left-on-rectangle, arrow-right-on-rectangle, arrow-small-{down,left,right,up}, minus-small, plus-small) correctly skip the :micro method and wrapper class.

Usage

# Using variant parameter
render PhlexIcons::Hero::Home.new(variant: :mini)
render PhlexIcons::Hero::Home.new(variant: :micro)

# Using variant-specific classes
render PhlexIcons::Hero::HomeMini.new
render PhlexIcons::Hero::HomeMicro.new

Changes

Split into two commits for easier review:

1. Code changes — Updated the generator to dynamically handle all 4 variants and updated the VARIANTS constant
2. Generated icons — Re-generated the Hero icon pack (640 new files, 325 modified)

[AliOsm]

Hello @bslobodin and thanks for your PR. I didn't add these when I implemented the gem as I thought these are a simple size change could be done using CSS. Is this correct or these have a special property?

[bslobodin]

👋 @AliOsm these are actually not just size variants you can replicate with CSS, I am certain they redrew every icon from scratch for each size.

A nice concrete example is ExclamationTriangle — the little warning dot at the bottom has a radius of 0.75 in the 24px solid, but mini and micro draw it with radius 1 to keep it visible at smaller sizes. Each variant has a different viewBox and completely redrawn paths, so they're genuinely separate icons rather than scaled copies.