Consumer `NodeId` public API.

[xStrom]

When updating AccessKit for Masonry in xilem#1699, one of the issues we ran into is that accesskit_consumer::NodeId doesn't have a public API for constructing it. For the time being we can temporarily work around that with unsafe code writing directly to memory, but this isn't a suitable long term solution.

NodeId has three methods new, with_same_tree, and to_components which are currently pub(crate). The easiest solution that would satisfy our needs is to change these to pub instead, for which I can also make the PR if that approach is acceptable.

If you have a strong reason to keep NodeId private to prevent construction of it, then that would also be good to know, so we could then discuss how Masonry's use of AccessKit for testing could continue in some other way.