Lint for likely-malformed reference style links

I often come across the situation in markdown when I accidentally write the following:

[useful text][reference]

[reference](<example.com>)

This, though it might look like a reference style link, is not:

[useful text][reference]

reference

The correct syntax is as follows:

[useful text][reference]

[reference]: <example.com>

useful text

Clippy could lint against this likely-malformed reference link.

Notes:

• This lint should only trigger if [reference](<link>) is on a line on its own (e.g. would be a valid reference target) and reference could be a valid reference key.
• This lint should not trigger if there is any escaping of the in-text part of the reference which makes it not a valid reference link, even if it renders [like][so] in the output.
• I don't think [bracketed][words] would ever be desired except in a code block about markdown, so this lint shouldn't have a lot of real-world false positives.

The issue names no files, tests, or entry points. Start by locating the lint registration and markdown parsing tests, then use the examples and notes to define the matching cases; done means detecting standalone likely reference targets while excluding escaped in-text references.