settings_ui: Handle enums with fields (#37945)

Closes #ISSUE

Adds handling for Enums with fields (i.e. not `enum Foo { Yes, No }`) in
Settings UI. Accomplished by creating default values for each element
with fields (in the derive macro), and rendering a toggle button group
with a button for each variant where switching the active variant sets
the value in the settings JSON to the default for the new active
variant.

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Conrad <conrad@zed.dev>

crates/settings/src/settings_ui_core.rs

@@ -108,10 +108,19 @@ impl<T: serde::Serialize> SettingsValue<T> {
 
 #[derive(Clone)]
 pub struct SettingsUiItemUnion {
-    pub options: Vec<SettingsUiEntry>,
+    /// Must be the same length as `labels` and `options`
+    pub defaults: Box<[serde_json::Value]>,
+    /// Must be the same length as defaults` and `options`
+    pub labels: &'static [&'static str],
+    /// Must be the same length as `defaults` and `labels`
+    pub options: Box<[Option<SettingsUiEntry>]>,
     pub determine_option: fn(&serde_json::Value, &App) -> usize,
 }
 
+// todo(settings_ui): use in ToggleGroup and Dropdown
+#[derive(Clone)]
+pub struct SettingsEnumVariants {}
+
 pub struct SettingsUiEntryMetaData {
     pub title: SharedString,
     pub path: SharedString,
@@ -136,6 +145,7 @@ pub enum SettingsUiItem {
     Single(SettingsUiItemSingle),
     Union(SettingsUiItemUnion),
     DynamicMap(SettingsUiItemDynamicMap),
+    // Array(SettingsUiItemArray), // code-actions: array of objects, array of string
     None,
 }