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/zed/src/zed/component_preview.rs

@@ -697,7 +697,7 @@ impl ComponentPreview {
             workspace.update(cx, |workspace, cx| {
                 let status_toast =
                     StatusToast::new("`zed/new-notification-system` created!", cx, |this, _cx| {
-                        this.icon(ToastIcon::new(IconName::GitBranchSmall).color(Color::Muted))
+                        this.icon(ToastIcon::new(IconName::GitBranchAlt).color(Color::Muted))
                             .action("Open Pull Request", |_, cx| {
                                 cx.open_url("https://github.com/")
                             })

crates/zed/src/zed/quick_action_bar/repl_menu.rs

@@ -212,7 +212,7 @@ impl QuickActionBar {
             .trigger_with_tooltip(
                 ButtonLike::new_rounded_right(element_id("dropdown"))
                     .child(
-                        Icon::new(IconName::ChevronDownSmall)
+                        Icon::new(IconName::ChevronDown)
                             .size(IconSize::XSmall)
                             .color(Color::Muted),
                     )