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/language_models/src/provider/cloud.rs

@@ -437,7 +437,7 @@ fn render_accept_terms(
         .style(ButtonStyle::Subtle)
         .icon(IconName::ArrowUpRight)
         .icon_color(Color::Muted)
-        .icon_size(IconSize::XSmall)
+        .icon_size(IconSize::Small)
         .when(thread_empty_state, |this| this.label_size(LabelSize::Small))
         .on_click(move |_, _window, cx| cx.open_url("https://zed.dev/terms-of-service"));