[wpilibNewCommands] Add get subsystem list function from CommandScheduler

[OhmV-IR]

In java, the subsystems are returned as an immutable set and in c++ they are returned as a const vector. No tests for these as there isn't testing framework to create mock subsystems in wpilibNewCommands tests, at least not that I'm aware of.

[github-actions]

This PR modifies commands. Please open a corresponding PR in Python Commands and include a link to this PR.

[KangarooKoala]

Is the intent to do a copy or return an object that is a readonly view? (Does the below work?)

Set<Subsystem> subsystems = CommandScheduler.getInstance().getAllRegisteredSubsystems();
Subsystem mySubsystem = new Subsystem() {};
CommandScheduler.getInstance.registerSubsystem(mySubsystem);
assertTrue(subsystems.contains(mySubsystem));

As it is right now, Java is a readonly view (the code snippet above will pass) while C++ is a copy (the C++ version of the above will not pass).

See #6475 for discussion of this feature and #6526 for the troubles encountered a year ago in getting the readonly view to work in C++.

[OhmV-IR]

robotpycommands2 pr: robotpy/robotpy-commands-v2#86

[OhmV-IR]

Ideally it will be a readonly view in c++, as that makes the most sense but might have to make both copies if it turns out that we can't get the readonly view to compile in cpp