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

Some headers were renamed, but others were split.

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

@@ -0,0 +1,20 @@
+// 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
+
+#include "PortsInternal.hpp"
+#include "wpi/hal/handles/HandlesInternal.hpp"
+#include "wpi/hal/handles/LimitedHandleResource.hpp"
+
+namespace wpi::hal {
+
+struct Counter {
+  uint8_t index;
+};
+
+extern LimitedHandleResource<HAL_CounterHandle, Counter, kNumCounters,
+                             HAL_HandleEnum::Counter>* counterHandles;
+
+}  // namespace wpi::hal