Lint for likely-malformed reference style links

[CAD97]

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.

[CAD97]

The original OP had an unrelated lint triggering on an invalid reference style link which should not (and does not) trigger for an actual reference style link.

This comment was me realizing my mistake.

[llogiq]

We could still try to detect those cases and issue a better warning message.

[CAD97]

What I had was [ref](link) instead of [ref]: link if you want to lint against it @llogiq. It feels like a common mistake (at least for me).

[CAD97]

I have re-purposed this issue to lint against near-miss reference style links.