Add ability to display backgrounds for inlay hints (#18010)

This PR adds the ability to display backgrounds for inlay hints within
the editor.

This is controlled by the new `inlay_hints.show_background` setting.
This setting defaults to `false`.

To enable the setting, add the following to your `settings.json`:

```json
{
  "inlay_hints": {
    "enabled": true,
    "show_background": true
  }
}
```

When enabled, the inlay hint backgrounds will use the `hint.background`
color from the theme.

| Disabled | Enabled |
|
--------------------------------------------------------------------------------------------------------------------------------------------------
|
--------------------------------------------------------------------------------------------------------------------------------------------------
|
| <img width="1624" alt="Screenshot 2024-09-17 at 4 21 53 PM"
src="https://github.com/user-attachments/assets/5534d09b-1e22-4c6f-9d82-314796ed7d22">
| <img width="1624" alt="Screenshot 2024-09-17 at 4 21 43 PM"
src="https://github.com/user-attachments/assets/6ec58cde-6115-4db4-be95-97c5f2f54b2d">
|

Related issues:

- #12485
- #17392

Release Notes:

- Added an `inlay_hints.show_background` setting to allow displaying
backgrounds for inlay hints in the editor.
  - This setting defaults to `false`.
- If enabled, the inlay hint backgrounds will use the `hint.background`
color from the theme.

crates/assistant/src/prompt_library.rs

@@ -921,10 +921,8 @@ impl PromptLibrary {
                                                     scrollbar_width: Pixels::ZERO,
                                                     syntax: cx.theme().syntax().clone(),
                                                     status: cx.theme().status().clone(),
-                                                    inlay_hints_style: HighlightStyle {
-                                                        color: Some(cx.theme().status().hint),
-                                                        ..HighlightStyle::default()
-                                                    },
+                                                    inlay_hints_style:
+                                                        editor::make_inlay_hints_style(cx),
                                                     suggestions_style: HighlightStyle {
                                                         color: Some(cx.theme().status().predictive),
                                                         ..HighlightStyle::default()