[cmd2] Move .cpp files to subproject source root (#8967)

This matches all of the other subprojects.

Signed-off-by: Zach Harel <zach@zharel.me>

commandsv2/src/main/native/cpp/NotifierCommand.cpp

@@ -0,0 +1,36 @@
+// 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.
+
+#include "wpi/commands2/NotifierCommand.hpp"
+
+#include <utility>
+
+using namespace wpi::cmd;
+
+NotifierCommand::NotifierCommand(std::function<void()> toRun,
+                                 wpi::units::second_t period,
+                                 Requirements requirements)
+    : m_toRun(toRun), m_notifier{std::move(toRun)}, m_period{period} {
+  AddRequirements(requirements);
+}
+
+NotifierCommand::NotifierCommand(NotifierCommand&& other)
+    : CommandHelper(std::move(other)),
+      m_toRun(other.m_toRun),
+      m_notifier(other.m_toRun),
+      m_period(other.m_period) {}
+
+NotifierCommand::NotifierCommand(const NotifierCommand& other)
+    : CommandHelper(other),
+      m_toRun(other.m_toRun),
+      m_notifier(wpi::Notifier(other.m_toRun)),
+      m_period(other.m_period) {}
+
+void NotifierCommand::Initialize() {
+  m_notifier.StartPeriodic(m_period);
+}
+
+void NotifierCommand::End(bool interrupted) {
+  m_notifier.Stop();
+}