Use more LSP data when falling back to regular completions label (#23909)

Closes https://github.com/zed-industries/zed/issues/23590
Closes https://x.com/steeve/status/1865129235536568555

Before:

<img width="773" alt="before"
src="https://github.com/user-attachments/assets/129a8d12-9298-4bf5-8f2d-b3292c2562bf"
/>


After:

<img width="768" alt="after"
src="https://github.com/user-attachments/assets/e0516fb3-b02a-48be-8923-63bba05fdb69"
/>


The list obviously needs some solution for the cut-off part of the
completion label, but this is the reality for all extensions'
completions too, so one step at a time.


Release Notes:

- Improved default completion label fallback