formatter: Support customization of link style preference

[askoufis]

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.

[rpaul-stripe]

The markdown-it library (which we use as a tokenizer) doesn't capture in the link token whether the link was written as a reference. But I don't think it would be too hard to add that to our markdown-it patch set. We would need to have it add the reference text as a meta property on the token, and then we could carry it over to the Markdoc AST and use it in the formatter.