Refactor completions (#42122)

This is progress toward multi-word snippets (including snippets with
prefixes containing symbols)

Release Notes:

- Removed `trigger` argument in `ShowCompletions` command

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/agent_ui/src/text_thread_editor.rs

@@ -478,7 +478,7 @@ impl TextThreadEditor {
                     editor.insert(&format!("/{name}"), window, cx);
                     if command.accepts_arguments() {
                         editor.insert(" ", window, cx);
-                        editor.show_completions(&ShowCompletions::default(), window, cx);
+                        editor.show_completions(&ShowCompletions, window, cx);
                     }
                 });
             });