Add command_mode action

src/actions.rs

@@ -71,6 +71,10 @@ pub fn goto_line_end_newline(state: &mut State) {
     state.set_error("goto_line_end_newline is unimplemented");
 }
 
+pub fn command_mode(state: &mut State) {
+    state.mode = Mode::Command(Default::default());
+}
+
 pub fn normal_mode(state: &mut State) {
     match state.mode {
         Mode::Insert(InsertState { append: true }) => {