Allow Shuffleboard state to be reset

[SamCarlberg]

Useful for unit tests

Alternatively, make ShuffleboardRoot and ShuffleboardInstance public and allow access to a default instance through the primary Shuffleboard class' API. Would behave similarly to NetworkTablesInstance.getDefault() and NetworkTablesInstance.create()

[Starlight220]

This exists:

allwpilib/wpilibc/src/main/native/cpp/shuffleboard/Shuffleboard.cpp

Lines 73 to 80 in 34e7849

	#ifndef __FRC_ROBORIO__
	namespace frc::impl {
	void ResetShuffleboardInstance() {
	GetInstanceHolder() = std::make_unique<detail::ShuffleboardInstance>(
	nt::NetworkTableInstance::GetDefault());
	}
	} // namespace frc::impl
	#endif

Question is how do we document this.

EDIT: Also, it's C++-only.