assistant: Add example streaming slash command (#20034)

This PR adds a `/streaming-example` slash command for the purposes of
showcasing streaming during development.

This slash command is only available to staff and isn't intended to be
shipped to the general public.

Release Notes:

- N/A

crates/assistant/src/slash_command.rs

@@ -31,6 +31,7 @@ pub mod now_command;
 pub mod project_command;
 pub mod prompt_command;
 pub mod search_command;
+pub mod streaming_example_command;
 pub mod symbols_command;
 pub mod tab_command;
 pub mod terminal_command;