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/ui/src/components/context_menu.rs

@@ -561,7 +561,7 @@ impl ContextMenu {
             action: Some(action.boxed_clone()),
             handler: Rc::new(move |_, window, cx| window.dispatch_action(action.boxed_clone(), cx)),
             icon: Some(IconName::ArrowUpRight),
-            icon_size: IconSize::XSmall,
+            icon_size: IconSize::Small,
             icon_position: IconPosition::End,
             icon_color: None,
             disabled: false,