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/search/src/buffer_search.rs

@@ -336,7 +336,7 @@ impl Render for BufferSearchBar {
                         this.child(
                             IconButton::new(
                                 "buffer-search-bar-toggle-search-selection-button",
-                                IconName::SearchSelection,
+                                IconName::Quote,
                             )
                             .style(ButtonStyle::Subtle)
                             .shape(IconButtonShape::Square)

crates/search/src/project_search.rs

@@ -2268,7 +2268,7 @@ impl Render for ProjectSearchBar {
                         .min_w_64()
                         .gap_1()
                         .child(
-                            IconButton::new("project-search-opened-only", IconName::FileSearch)
+                            IconButton::new("project-search-opened-only", IconName::FolderSearch)
                                 .shape(IconButtonShape::Square)
                                 .toggle_state(self.is_opened_only_enabled(cx))
                                 .tooltip(Tooltip::text("Only Search Open Files"))