Update `unified-language-server`

[remcohaszing]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

This updates unified-language-server to version 3. See https://github.com/unifiedjs/unified-language-server/releases/tag/3.0.0 for details.

[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.