Migration assistance for operators

[arbrandes]

Description

In order to help ease the migration to frontend-base, investigate ways that existing env.config.jsx files 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 one or more env.config.jsx files into a single site.config.tsx one
• A script for site operators that attempts to convert an env.config.jsx into an equivalent site.config.tsx, asking the user for input when and if required
• Documentation and/or a script for plugin authors that helps convert an existing frontend plugin into a frontend-base app based on existing env.config.jsx slot configuration