Add formal specification of atjson format

[nichtich]

One lesson I learned in decades of working with data: without specification and validation, eventually data will not conform to an expected data format anymore. If atjson format is going to be used beyond its current software, it needs a formal specification.

I've started a JSON Schema in my specification branch. The format is quite simple so the schema is not complex. One option question I stumbled upon is whether the end position is optional (I don't think so)?

To validate atjson documents against the schema I am looking for actual examples and use cases. Validation should best be added to unit tests at least.