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/prompt_command.rs

@@ -21,6 +21,10 @@ impl SlashCommand for PromptSlashCommand {
         "Insert prompt from library".into()
     }
 
+    fn icon(&self) -> IconName {
+        IconName::Library
+    }
+
     fn menu_text(&self) -> String {
         self.description()
     }