feat: allow applications to trigger the shortcut help dialog (#290)

This will be be used on an interm basis in MakeCode.

Author: microbit-matt-hillsdon

src/index.ts

@@ -137,6 +137,13 @@ export class KeyboardNavigation {
     this.navigationController.dispose();
   }
 
+  /**
+   * Toggle visibility of a help dialog for the keyboard shortcuts.
+   */
+  toggleShortcutDialog(): void {
+    this.navigationController.shortcutDialog.toggle();
+  }
+
   /**
    * Update the theme to match the selected glow colour to the cursor
    * colour.