lsp: documentHighlight should highlight metadata keys for multi line strings

[charlieegan3]

Note that the multi-line value is not highlighted.

Screen.Recording.2025-09-09.at.11.10.34.mov

[anderseknert]

Lol, IIRC my intention wasn't even that clicking the value would highlight the attribute — only clicking the attribute itself. But seeing how it works in your video, I actually like it! So yeah, let's make it work for multi-line values too. Not sure what the best approach would be... but I'm sure we can figure it out.