Display default prompts more elaborately (#16471)

Show them under `User` role instead of a `System` one, and insert them
expanded.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -124,6 +124,7 @@ impl SlashCommandCompletionProvider {
                                                             &command_name,
                                                             &[],
                                                             true,
+                                                            false,
                                                             workspace.clone(),
                                                             cx,
                                                         );
@@ -208,6 +209,7 @@ impl SlashCommandCompletionProvider {
                                                             &command_name,
                                                             &completed_arguments,
                                                             true,
+                                                            false,
                                                             workspace.clone(),
                                                             cx,
                                                         );