Added bracketed path navigation to variant_tree

Example tree:
    key1 "abc"
    key1 "data1" {
        kx1 "foo"
        kx3 "xxx"
        kx3 "data2" {
            key2 123 {
                a 1
                b "ok"
            }
        }
    }

    auto res = tree.get_child_optional("key1[data1]/[data2]/key2")

This will return the child tree of the
{key1, "data1"} / {kx3, "data2"} / {key2, 123} entry