lsp: Add support for label_details in completions (#13043)

This fixes an issue reported by @Spoutnik97 in
https://github.com/zed-industries/zed/issues/12711#issuecomment-2163785111
- vtsls returns auxiliary docs via .label_details and not plain .details
field.

Release Notes:

- Improved quality of auxiliary details in completions returned by VTSLS