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>

assets/keymaps/linux/emacs.json

@@ -98,6 +98,13 @@
       "ctrl-n": "editor::ContextMenuNext"
     }
   },
+  {
+    "context": "Editor && showing_signature_help && !showing_completions",
+    "bindings": {
+      "ctrl-p": "editor::SignatureHelpPrevious",
+      "ctrl-n": "editor::SignatureHelpNext"
+    }
+  },
   {
     "context": "Workspace",
     "bindings": {