feat: reverse_iterator support

[sxrzh]

Adds reverse_iterator support for YAML::Node, including:

• Node::reverse_iterator and Node::const_reverse_iterator
• Node::rbegin() and Node::rend(), including const versions

I didn't do the same for node_iterator, for I think node iterators won't be used directly and there're no systematic tests for node iterators.

Currently iterator_base and node_iterator_base are incompatible with std::reverse_iterator, since their operator->() function returns a proxy object, which could cause dangling pointer with std::reverse_iterator. So I implemented a customized reverse_iterator_base wrapper in yaml-cpp/node/detail/reverse_iterator.h .

[sxrzh]

So I implemented a customized reverse_iterator_base wrapper in yaml-cpp/node/detail/reverse_iterator.h .

Note that this does not meet the requirements for ReversibleContainer. Since the current YAML::Node is also far away from Container (maybe it is never intended to be), I think this is acceptable.