[ty] Add type as detail to completion items (#20047)
## Summary
@BurntSushi was so kind as to find me an easy task to do some coding
before I'm off to PTO.
This PR adds the type to completion items (see the gray little text at
the end of a completion item).
https://github.com/user-attachments/assets/c0a86061-fa12-47b4-b43c-3c646771a69d
