Add namepath to assignments in JSON dump

Hi @casey Please consider adding namepath to variables in the JSON dump. It exists for recipes but not for variables I am currently walking the JSON output to reconstruct the namepath manually - it works but would be nice if it was in the dump. ### Use case I use this to test that in recipe body invocations of just are resolvable. Just invocations in a recipe need to use `-f` to avoid breaking if I am in a directory that has its own just file (e.g. invoking my personal justfile when in a project directory that has its own justfile) I define _just_call_* variables ``` _mod := "docker::compose" _just_call_pick_services := "just --justfile " + justfile() + "{{ _mod }}::_pick-services" ``` then in my tests apply two-step verification: 1. ``just --evaluate <module>::<var>`` resolves _just + _mod into a concrete command 2. Execute that command with JUST_DRY_RUN=true to prove the target recipe is reachable