theme: Add support for setting light/dark icon themes (#24702)

This PR adds support for configuring both a light and dark icon theme in
`settings.json`.

In addition to accepting just an icon theme name, the `icon_theme` field
now also accepts an object in the following form:

```jsonc
{
  "icon_theme": {
    "mode": "system",
    "light": "Zed (Default)",
    "dark": "Zed (Default)"
  }
}
```

Both `light` and `dark` are required, and indicate which icon theme
should be used when the system is in light mode and dark mode,
respectively.

The `mode` field is optional and indicates which icon theme should be
used:
- `"system"` - Use the icon theme that corresponds to the system's
appearance.
- `"light"` - Use the icon theme indicated by the `light` field.
- `"dark"` - Use the icon theme indicated by the `dark` field.

Closes https://github.com/zed-industries/zed/issues/24695.

Release Notes:

- Added support for configuring both a light and dark icon theme and
switching between them based on system preference.

crates/workspace/src/workspace.rs

@@ -1082,6 +1082,7 @@ impl Workspace {
                 *SystemAppearance::global_mut(cx) = SystemAppearance(window_appearance.into());
 
                 ThemeSettings::reload_current_theme(cx);
+                ThemeSettings::reload_current_icon_theme(cx);
             }),
             cx.on_release(move |this, cx| {
                 this.app_state.workspace_store.update(cx, move |store, _| {