settings: Use a derive macro for refine (#38451)

When we refactored settings to not pass JSON blobs around, we ended up
needing
to write *a lot* of code that just merged things (like json merge used
to do).

Use a derive macro to prevent typos in this logic.

Release Notes:

- N/A

crates/onboarding/src/ai_setup_page.rs

@@ -265,7 +265,7 @@ pub(crate) fn render_ai_setup_page(
 
                     let fs = <dyn Fs>::global(cx);
                     update_settings_file(fs, cx, move |settings, _| {
-                        settings.disable_ai = Some(enabled);
+                        settings.disable_ai = Some(enabled.into());
                     });
                 },
             )