This is a Mautic-Plugin to demonstrate how 3rd party or custom GrapesJS plugins can be added to Mautic's editor.
You can use this plugin as skeleton to start extending and customizing the GrapeJS editor for your needs.
Basically any plugin bundle is able to add custom GrapesJS plugins using a global window-object for the registration.
Read how to do so in the official developer docs: Creating a Plugin for GrapesJS.