Modifying MENTION_START regex to work on older MobileSafari versions

Fixes RichText causes errors on WebKit | Invalid regex #5589

This regex performs the same function as the one used before, but does not make use of negative lookbehinds and therefore should work on older MobileSafari versions.

src/mixins/richEditor/index.js

@@ -11,8 +11,9 @@ import stripTags from 'striptags'
 import Vue from 'vue'
 
 // Referenced from public function getMentions(): https://github.com/nextcloud/server/blob/master/lib/private/Comments/Comment.php
-// Beginning or whitespace. Non-capturing group within word boundary
-const MENTION_START = /\B(?<![^a-z0-9_\-@.'\s])/.source
+// Modified to work on MobileSafari <16.4 (see issue #5589: https://github.com/nextcloud-libraries/nextcloud-vue/issues/5589)
+// Beginning or whitespace.
+const MENTION_START = /[a-z0-9_\-@.'\s]\B/.source
 // Capturing groups like: @user-id, @"guest/abc16def", @"federated_user/user-id", @"user-id with space"
 const MENTION_SIMPLE = /(@[a-z0-9_\-@.']+)/.source
 const MENTION_GUEST = /@&quot;guest\/[a-f0-9]+&quot;/.source