[hal] Consistently use .hpp for C++ header content

Some headers were renamed, but others were split.

hal/src/main/native/sim/NotifierInternal.hpp

@@ -0,0 +1,13 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+namespace wpi::hal {
+void PauseNotifiers();
+void ResumeNotifiers();
+void WakeupNotifiers();
+void WaitNotifiers();
+void WakeupWaitNotifiers();
+}  // namespace wpi::hal