lsp: Complete overloaded signature help implementation (#33199)

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>

crates/project/src/lsp_store.rs

@@ -6504,7 +6504,6 @@ impl LspStore {
                     .await
                     .into_iter()
                     .flat_map(|(_, actions)| actions)
-                    .filter(|help| !help.label.is_empty())
                     .collect::<Vec<_>>()
             })
         }