Convert module to well-formatted ES modules

[jswalden]

First, autoformat all the code. Second, change the key-splitter used in callback.json so that no keys turn into numbers, when this file is made a plain JS module. Third, do the absolute minimum to convert everything to ES modules, because that plays better with Intellisense-style stuff.

[josephdadams]

@jswalden you have maintain access now to this repo, have at it