Add CLI option to commands dbmate create and dbmate drop to use another database than postgres to create/drop the database

[maximilian-hammerl]

Supabase uses the postgres database as the default database. Therefore, dbmate create and dbmate drop fail, because you cannot drop the currently connected database and dbmate always uses the postgres database to create/drop a database. (I've already run into this problem: #747)

It would be great, if one could specify another database than postgres for dbmate to use with dbmate create and dbmate drop.

(When running db reset using the Supabase CLI, it uses the template1 database for dropping and creating the database: https://github.com/supabase/cli/blob/develop/internal/db/reset/reset.go#L154)

(I am open to implementing this if there is positive feedback on this feature request.)