Raise helpful error if trying to set URL on Avram instead of your database

If you do:

```crystal
Avram.configure do |settings|
settings.url = # ...
end
```

It will throw an error at compile time because that setting doesn't exist. You need to set it on a database class.

This rarely happens since Lucky sets it up for you, but if/when you need to use Avram by itself this can be confusing when setting things up.

Should have an error like:

> You can't set the database URL on Avram. Instead set it on a database class

And include how to create a database class

Reproduce the Avram.configure example with settings.url and inspect the configuration and database-class entry points involved in the compile-time error. Done means setting the URL on Avram produces the requested helpful message, including how to create a database class, while database-class configuration remains usable.

Written by the indexing model from the issue text.