Add special highlight key for LSP-provided document links.

[senekor]

I was recently confused by the syntax highlighting of a Cargo.toml file when using Tombi LSP. Some stuff was light blue, instead of the usual colors of toml files.

Once I realized these were document links, which I could follow with gf, it made sense to me. But the light blue doesn't scream "I'm a link!" to me. So I thought it might be good to add "underlined" to that highlighting. However, that also adds the underline to things that are clearly links, like URLs in comments that start with https://. Adding underlines to those seems very noisy to me.

That made me think, it would be nice to have a special highlight key for LSP-provided document links, to make them stand out even more. If that was possible, I would propose to add underlines to them for the Kanagawa theme, while keeping the regular "markup.link.url" the same.