Path expressions substitution

[zakalibit]

Path expressions are used to write out a path through the object graph. They appear in two places; in substitutions, like ${foo.bar}, and as the keys in objects like { foo.bar : 42 }.

As well this should work as value concatenation like:
def: "default"
opt :{
name: "${def}-name"
}

that should result in
opt :{
name: "default-name"
}

As well if the ${var} is not found in the current file, try to resolve from the environment.