edtlib: move Node

This is just moving the class definition higher in the file. I am
reordering the classes to make it possible to type annotate the module
in a more readable way.

Git might make the diff look bigger than it really is.
To verify this is just moving code, use 'git diff --minimal'.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>