[commands] Add a method to get registered subsystems

[spacey-sooty]

Still needs C++, most likely to be done with
https://en.cppreference.com/w/cpp/ranges/keys_view depending on compatibility with llvm::DenseMap and C++ 20 support.

Resolves #6475

[spacey-sooty]

I'm getting a lot of errors with keys_view I don't think its going to work with llvm::DenseMap

[calcmogul]

What's the error?

[spacey-sooty]

output.txt

[KangarooKoala]

std::ranges::key_view is an alias of std::ranges::elements_view, which requires that the container (in our case, wpi::DenseMap) satisfy std::ranges::view and be tuple-like (Or maybe the element type (in this case wpi::detail::DenseMapPair) be tuple-like?), which involves writing specializations of std::tuple_size and std::tuple_element. There's also something about std::get, but I'm not sure if that's going to be a problem.