formatter: Support customization of link style preference

Overview

Markdoc supports reference links, however the formatter chooses to format these into inline links.

Example

[A link][link]

[link]: www.example.com

is formatted as

[A link](www.example.com)

Discussion

I feel like this stylistic formatting choice should be opt-in, rather than enabled by default, if it even warrants being configurable. I'd guess that the list of people that want to configure this stylistic choice is very small, and given that prettier does not allow customization of this stylistic choice, I don't even think it should be configurable, but rather all styles of links should just be left alone.