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/assistant_tools/src/edit_file_tool.rs

@@ -857,7 +857,7 @@ impl ToolCard for EditFileToolCard {
                     )
                     .child(
                         Icon::new(IconName::ArrowUpRight)
-                            .size(IconSize::XSmall)
+                            .size(IconSize::Small)
                             .color(Color::Ignored),
                     ),
             )

crates/assistant_tools/src/find_path_tool.rs

@@ -257,7 +257,7 @@ impl ToolCard for FindPathToolCard {
 
                         Button::new(("path", index), button_label)
                             .icon(IconName::ArrowUpRight)
-                            .icon_size(IconSize::XSmall)
+                            .icon_size(IconSize::Small)
                             .icon_position(IconPosition::End)
                             .label_size(LabelSize::Small)
                             .color(Color::Muted)

crates/assistant_tools/src/web_search_tool.rs

@@ -45,7 +45,7 @@ impl Tool for WebSearchTool {
     }
 
     fn icon(&self) -> IconName {
-        IconName::Globe
+        IconName::ToolWeb
     }
 
     fn input_schema(&self, format: LanguageModelToolSchemaFormat) -> Result<serde_json::Value> {
@@ -177,7 +177,7 @@ impl ToolCard for WebSearchToolCard {
                             .label_size(LabelSize::Small)
                             .color(Color::Muted)
                             .icon(IconName::ArrowUpRight)
-                            .icon_size(IconSize::XSmall)
+                            .icon_size(IconSize::Small)
                             .icon_position(IconPosition::End)
                             .truncate(true)
                             .tooltip({