settings_ui: Add line number settings controls (#15310)

This PR adds settings controls for the line numbers and relative line
numbers settings.

Release Notes:

- N/A

crates/editor/src/editor_settings.rs

@@ -305,7 +305,7 @@ pub struct ScrollbarContent {
 }
 
 /// Gutter related settings
-#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, Default, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 pub struct GutterContent {
     /// Whether to show line numbers in the gutter.
     ///