Expose `Node` to api consumers

[solonovamax]

It would be nice if the expression function which returns a Node could be exposed to api consumers. My specific usecase is: I want to write a simply highlighting system for the expressions, and being able to recurse through the node tree would be quite useful for this.

Additionally, if the java version is upgraded, then perhaps the Node interface could be marked as sealed, as the different node types cannot be changed by api consumers.