[commands] Change grouping decorator impl to flatten nested group structures (#3335)

wpilibNewCommands/src/main/native/include/frc2/command/SequentialCommandGroup.h

@@ -84,6 +84,15 @@ class SequentialCommandGroup
 
   bool RunsWhenDisabled() const override;
 
+  SequentialCommandGroup BeforeStarting(
+      std::function<void()> toRun,
+      wpi::ArrayRef<Subsystem*> requirements = {}) &&
+      override;
+
+  SequentialCommandGroup AndThen(std::function<void()> toRun,
+                                 wpi::ArrayRef<Subsystem*> requirements = {}) &&
+      override;
+
  private:
   void AddCommands(std::vector<std::unique_ptr<Command>>&& commands) final;