fix(isSlug): change charsetRegex variable to const for better immutability

Author: Shrawak

src/lib/isSlug.js

@@ -1,6 +1,6 @@
 import assertString from './util/assertString';
 
-let charsetRegex = /^[a-z0-9](?!.*[-_]{2,})(?:[a-z0-9_-]*[a-z0-9])?$/;
+const charsetRegex = /^[a-z0-9](?!.*[-_]{2,})(?:[a-z0-9_-]*[a-z0-9])?$/;
 
 export default function isSlug(str) {
   assertString(str);