Migration assistance for plugin authors

[arbrandes]

Description

In order to help ease the migration to frontend-base, investigate ways that existing frontend plugins can be re-used, then select and implement the ones that are judged to be sufficient. This can take the form of:

• Specific documentation on how to convert existing plugins into frontend-base apps
• A script that helps convert an existing plugin into a frontend-base app based on existing code and env.config.jsx slot configuration