[commands] Revert SubsystemBase deprecation/removal (#5634)

wpilibNewCommands/src/main/native/cpp/frc2/command/SubsystemBase.cpp

@@ -12,4 +12,57 @@
 
 using namespace frc2;
 
-SubsystemBase::SubsystemBase() {}
+SubsystemBase::SubsystemBase() {
+  wpi::SendableRegistry::AddLW(this, GetTypeName(*this));
+  CommandScheduler::GetInstance().RegisterSubsystem({this});
+}
+
+void SubsystemBase::InitSendable(wpi::SendableBuilder& builder) {
+  builder.SetSmartDashboardType("Subsystem");
+  builder.AddBooleanProperty(
+      ".hasDefault", [this] { return GetDefaultCommand() != nullptr; },
+      nullptr);
+  builder.AddStringProperty(
+      ".default",
+      [this]() -> std::string {
+        auto command = GetDefaultCommand();
+        if (command == nullptr) {
+          return "none";
+        }
+        return command->GetName();
+      },
+      nullptr);
+  builder.AddBooleanProperty(
+      ".hasCommand", [this] { return GetCurrentCommand() != nullptr; },
+      nullptr);
+  builder.AddStringProperty(
+      ".command",
+      [this]() -> std::string {
+        auto command = GetCurrentCommand();
+        if (command == nullptr) {
+          return "none";
+        }
+        return command->GetName();
+      },
+      nullptr);
+}
+
+std::string SubsystemBase::GetName() const {
+  return wpi::SendableRegistry::GetName(this);
+}
+
+void SubsystemBase::SetName(std::string_view name) {
+  wpi::SendableRegistry::SetName(this, name);
+}
+
+std::string SubsystemBase::GetSubsystem() const {
+  return wpi::SendableRegistry::GetSubsystem(this);
+}
+
+void SubsystemBase::SetSubsystem(std::string_view name) {
+  wpi::SendableRegistry::SetSubsystem(this, name);
+}
+
+void SubsystemBase::AddChild(std::string name, wpi::Sendable* child) {
+  wpi::SendableRegistry::AddLW(child, GetSubsystem(), name);
+}