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/assistant.rs

@@ -51,6 +51,7 @@ use std::sync::Arc;
 pub(crate) use streaming_diff::*;
 use util::ResultExt;
 
+use crate::slash_command::streaming_example_command;
 use crate::slash_command_settings::SlashCommandSettings;
 
 actions!(
@@ -468,6 +469,19 @@ fn register_slash_commands(prompt_builder: Option<Arc<PromptBuilder>>, cx: &mut
     })
     .detach();
 
+    cx.observe_flag::<streaming_example_command::StreamingExampleSlashCommandFeatureFlag, _>({
+        let slash_command_registry = slash_command_registry.clone();
+        move |is_enabled, _cx| {
+            if is_enabled {
+                slash_command_registry.register_command(
+                    streaming_example_command::StreamingExampleSlashCommand,
+                    false,
+                );
+            }
+        }
+    })
+    .detach();
+
     update_slash_commands_from_settings(cx);
     cx.observe_global::<SettingsStore>(update_slash_commands_from_settings)
         .detach();