Redesign and clean up all icons across Zed (#35856)

- [x] Clean up unused and old icons
- [x] Swap SVG for all in-use icons with the redesigned version
- [x] Document guidelines

Release Notes:

- N/A

crates/zeta/src/onboarding_modal.rs

@@ -141,7 +141,7 @@ impl Render for ZedPredictModal {
                     )),
             )
             .child(h_flex().absolute().top_2().right_2().child(
-                IconButton::new("cancel", IconName::X).on_click(cx.listener(
+                IconButton::new("cancel", IconName::Close).on_click(cx.listener(
                     |_, _: &ClickEvent, _window, cx| {
                         onboarding_event!("Cancelled", trigger = "X click");
                         cx.emit(DismissEvent);