Adjust design of the slash command picker (#19973)

This PR removes the quote selection icon button from the footer and adds
it in the picker, and adds an icon field to each command entry. Final
result looks like:


https://github.com/user-attachments/assets/d177f1c1-b6f6-4652-9434-f6291b279e34

Release Notes:

- N/A

crates/assistant/src/slash_command/project_command.rs

@@ -24,7 +24,8 @@ use std::{
     ops::DerefMut,
     sync::{atomic::AtomicBool, Arc},
 };
-use ui::{BorrowAppContext as _, IconName};
+
+use ui::prelude::*;
 use workspace::Workspace;
 
 pub struct ProjectSlashCommand {
@@ -50,6 +51,10 @@ impl SlashCommand for ProjectSlashCommand {
         "Generate a semantic search based on context".into()
     }
 
+    fn icon(&self) -> IconName {
+        IconName::Folder
+    }
+
     fn menu_text(&self) -> String {
         self.description()
     }