Emit a warning for duplicate link labels that differ

[fowles]

I have a setup where I am concatenating several different .md files for reasons. I want to be able to use link labels in each of the sub things and then have the linter warn me if any of the duplicate labels have different URL targets but otherwise accept that duplicates are OK.

https://github.com/DavidAnson/markdownlint/blob/main/doc/md053.md is related but slightly too strict.