MotherDuck adapter

Add an adapter, based on the existing DuckDB adapter, for [MotherDuck](https://motherduck.com/) (cloud-hosted serverless DuckDB). TODO: - [x] Basic functionality with `:DB md: [query]` and `:DB md:[database] [query]` - [x] List tables for basic integration with [vim-dadbod-ui](https://github.com/kristijanhusak/vim-dadbod-ui) - [ ] Add documentation - [ ] ???