Node export should be removed from @types/xast

[remcohaszing]

@types/xast re-exports the unist.Node type. However, unist.Node is unrelated to xast, except that xast.Doctype extends it.

I think this leads to confusion, causing for example syntax-tree/xastscript#8.

It should probably be removed, but that would be a breaking. Should it be deprecated instead?

[ChristianMurphy]

@remcohaszing fair point, let's start with deprecating it, and we can remove it in the next major