settings_ui: Add dynamic setting fields (#40443)

Closes #ISSUE

Includes the start of how we can get rid of most of the `.unimplemented`
"Edit in JSON" buttons in the settings UI. For now only Theme selection
is implemented, follow ups will add more settings

Release Notes:

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

crates/settings/src/settings_content/theme.rs

@@ -125,7 +125,18 @@ fn default_buffer_font_weight() -> Option<FontWeight> {
 }
 
 /// Represents the selection of a theme, which can be either static or dynamic.
-#[derive(Clone, Debug, Serialize, Deserialize, JsonSchema, MergeFrom, PartialEq, Eq)]
+#[derive(
+    Clone,
+    Debug,
+    Serialize,
+    Deserialize,
+    JsonSchema,
+    MergeFrom,
+    PartialEq,
+    Eq,
+    strum::EnumDiscriminants,
+)]
+#[strum_discriminants(derive(strum::VariantArray, strum::VariantNames, strum::FromRepr))]
 #[serde(untagged)]
 pub enum ThemeSelection {
     /// A static theme selection, represented by a single theme name.
@@ -167,7 +178,18 @@ pub enum IconThemeSelection {
 ///
 /// `System` will select the theme based on the system's appearance.
 #[derive(
-    Debug, PartialEq, Eq, Clone, Copy, Default, Serialize, Deserialize, JsonSchema, MergeFrom,
+    Debug,
+    PartialEq,
+    Eq,
+    Clone,
+    Copy,
+    Default,
+    Serialize,
+    Deserialize,
+    JsonSchema,
+    MergeFrom,
+    strum::VariantArray,
+    strum::VariantNames,
 )]
 #[serde(rename_all = "snake_case")]
 pub enum ThemeMode {