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 |
|
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|
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|
| <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/language/src/language_settings.rs

@@ -741,6 +741,14 @@ pub struct InlayHintSettings {
     /// Default: true
     #[serde(default = "default_true")]
     pub show_other_hints: bool,
+    /// Whether to show a background for inlay hints.
+    ///
+    /// If set to `true`, the background will use the `hint.background` color
+    /// from the current theme.
+    ///
+    /// Default: false
+    #[serde(default)]
+    pub show_background: bool,
     /// Whether or not to debounce inlay hints updates after buffer edits.
     ///
     /// Set to 0 to disable debouncing.