feat: Add `underline` icon

icons/underline.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24"><path d="M6,3v7a6,6,0,0,0,6,6h0a6,6,0,0,0,6-6V3" fill="none" stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="2"/><line x1="4" y1="21" x2="20" y2="21" fill="none" stroke="#000" stroke-linecap="round" stroke-linejoin="round" stroke-width="2"/></svg>
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