New rule to enforce the type of reference link to use

[waldyrious]

Commonmark defines three types of reference links:

• full, e.g. [bar][foo]
• collapsed, e.g. [foo][]
• shortcut , e.g. [foo]

All of these refer to a link defined elsewhere as [foo]: https://example.com.

There's currently a PR (#845) implementing a rule to select which style of link to use: inline, reference, or consistent. To avoid overloading that rule, and following the recommendation here, I'm opening this issue to request a separate rule specifically for reference links.

I propose the values to be either consistent or an array of the allowable types of reference links (full, collapsed, shortcut).