refactor: externalize slugify function from mixin

src/mixins/slugify.js

+export function slugify(string) {
+  return string
+    .toLowerCase()
+    .replace(/\s+/g, '-') // Replace spaces with -
+    .replace(/[^\w-]+/g, '') // Remove all non-word chars
+    .replace(/--+/g, '-') // Replace multiple - with single -
+    .replace(/^-+/, '') // Trim - from start of text
+    .replace(/-+$/, '') // Trim - from end of text
+}
+
 export default {
   methods: {
-    slugify: function (title) {
-      return title
-        .toLowerCase()
-        .replace(/\s+/g, '-') // Replace spaces with -
-        .replace(/[^\w-]+/g, '') // Remove all non-word chars
-        .replace(/--+/g, '-') // Replace multiple - with single -
-        .replace(/^-+/, '') // Trim - from start of text
-        .replace(/-+$/, '') // Trim - from end of text
-    },
+    slugify,
   },
 }