Improve the installer to account for an existing multi-existing db configuration

[dhh]

With 1.0.3, we're modifying the config/database.yml file automatically. That works well for new out-of-the-box Rails apps. But if you've already modified that file with a multi-DB setup, or otherwise, we'll be overwriting that. This needs to be fixed before Rails 8, as both Solid Cache and Solid Queue will need to be configured out of the box.

[dhh]

If you wanted to take a stab at this @fractaledmind, here's the issue to track it.

[dhh]

Solved via 0a36dec and rails/rails@cae25f3