Consider making path to node readily available in Model

[davidmorgan]

Per this comment

https://github.com/dart-lang/macros/pull/68/files#r1759229776

it's really convenient for some use cases if a node in the model knows its own location. For example, that would mean a field in a class knows how it can be referred to from anywhere.

One option would be to change the schema to make that information explicit in the model.

But I wonder if we can cheat. When we instantiate a Map from a JsonBufferBuilder, it's always either the root map or it's in the context of another Map or List lookup. A Map is just a pointer into JsonBufferBuilder, I wonder if we could add also a pointer to the parent, and so provide a fast Model.pathTo(node) that uses the parent pointers to compute the path from root to the node.