allow interacting with key value pairs via subscription

[FlorianLudwig]

example usage:

import json5.loader
import json5.dumper


model = json5.loader.loads('{foo: "bar" /*a comment */, "a": 1}', loader=json5.loader.ModelLoader())
model.value["foo"] = json5.dumper.modelize("not-bar")

print(json5.dumper.dumps(model, dumper=json5.dumper.ModelDumper()))