feat(button): Add black variant (#34)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

src/runtime/presets/default.ts

@@ -33,6 +33,7 @@ const button = {
     }, {}),
     secondary: 'border border-transparent text-primary-700 bg-primary-100 hover:bg-primary-200 disabled:bg-primary-100 focus:ring-2 focus:ring-offset-2 focus:ring-primary-500',
     white: 'shadow-sm border u-border-gray-300 u-text-gray-700 u-bg-white hover:u-bg-gray-50 disabled:u-bg-white focus:ring-2 focus:ring-offset-2 focus:ring-primary-500',
+    black: 'shadow-sm border border-transparent u-text-white u-bg-gray-800 hover:u-bg-gray-900 disabled:u-bg-gray-800 focus:ring-2 focus:ring-offset-2 focus:ring-primary-500',
     transparent: 'border border-transparent u-text-gray-500 hover:u-text-gray-700 focus:u-text-gray-700 disabled:hover:u-text-gray-500',
     link: 'border border-transparent text-primary-500 hover:text-primary-700 focus:text-primary-700'
   },