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/delta_command.rs

@@ -10,6 +10,7 @@ use gpui::{Task, WeakView, WindowContext};
 use language::{BufferSnapshot, LspAdapterDelegate};
 use std::sync::{atomic::AtomicBool, Arc};
 use text::OffsetRangeExt;
+use ui::prelude::*;
 use workspace::Workspace;
 
 pub(crate) struct DeltaSlashCommand;
@@ -27,6 +28,10 @@ impl SlashCommand for DeltaSlashCommand {
         self.description()
     }
 
+    fn icon(&self) -> IconName {
+        IconName::Diff
+    }
+
     fn requires_argument(&self) -> bool {
         false
     }