Add MotherDuck adapter

[Doekeb]

Add an adapter, based on the existing DuckDB adapter, for MotherDuck (cloud-hosted serverless DuckDB).

This also improves the output format of the existing DuckDB adapter to use duckbox instead of columns.

For example:
Before:

After:

Note in particular the presence of data types in column headers.

TODO:

• Basic functionality with :DB md: [query] and :DB md:[database] [query]
• List tables for basic integration with vim-dadbod-ui
• Add documentation

[Doekeb]

@tpope I was mostly able to figure out the interface, but I'm not completely sure on what canonicalize is supposed to do or what it's used for. Right now I have it set up to return the URL unchanged. MotherDuck CLI assumes a token is stored as an environment variable so the connection URL is extremely simple.

Aside from the documentation TODO, I'm interested to hear if you think anything else is missing!