Regenerate completion labels on resolve (#21521)

Closes https://github.com/zed-industries/zed/issues/21516

Technically, this is an LSP violation from `vtsls`, but seems that it's
not going to be fixed adequately on that side, see
https://github.com/yioneko/vtsls/issues/213 for more context.
So, we have to accommodate at least for now.

Release Notes:

- Fixed completion item labels not being updated after the resolve for
non-LSP compliant servers