This PR revives zed-industries/zed#27818 and aims to complete the
partially implemented overloaded signature help feature.

The first commit is a rebase of zed-industries/zed#27818, and the
subsequent commit addresses all review feedback from the original PR.

Now the overloaded signature help works like


https://github.com/user-attachments/assets/e253c9a0-e3a5-4bfe-8003-eb75de41f672

Closes #21493

Release Notes:

- Implemented signature help for overloaded items. Additionally, added a
support for rendering signature help documentation.

---------

Co-authored-by: Fernando Tagawa <tagawafernando@gmail.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>
Co-authored-by: Kirill Bulatov <kirill@zed.dev>