* Add `dockerfile.stages.*.extra_directories` as an optional list of
strings.
* Allow `php.phpstan` and `php.phpcs` to be `false` or a Collection so
their configuration files can not be generated.
* Add `php.phpunit` and allow it to be set to `false` so its
configuration files can not be generated.
* Add `database.extra_databases`
* Add `php.phpstan.baseline` as an optional boolean.
* Add `node.version` as a string.
Use the new database credentials by default.
The `experimental.useNewDatabaseCredentials` feature flag can be removed
once it's been removed from all `build.yaml` files.
