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_slash_command/src/assistant_slash_command.rs

@@ -62,6 +62,9 @@ pub type SlashCommandResult = Result<BoxStream<'static, Result<SlashCommandEvent
 
 pub trait SlashCommand: 'static + Send + Sync {
     fn name(&self) -> String;
+    fn icon(&self) -> IconName {
+        IconName::Slash
+    }
     fn label(&self, _cx: &AppContext) -> CodeLabel {
         CodeLabel::plain(self.name(), None)
     }