Port shadows module to plugin

src/plugins/shadows.js

@@ -0,0 +1,14 @@
+import _ from 'lodash'
+
+export default function () {
+  return function ({ addUtilities, config, e }) {
+    const utilities = _.fromPairs(_.map(config('shadows'), (value, modifier) => {
+      const className = modifier === 'default' ? 'shadow' : `shadow-${modifier}`
+      return [`.${className}`, {
+        'box-shadow': value,
+      }]
+    }))
+
+    addUtilities(utilities, config('modules.shadows'))
+  }
+}