Create a library to support programmatic processing of item files

[erikhofer]

In principle, you can already use a simple YAML parser to work with the files.
This is, however, not very convenient. A small library that enriches the data model would help with making the data more accessible.

We will start with a JavaScript (TypeScript) library and may add a JVM implementation later. Other platform would probably need to be supported by the community.

The library should somehow sit on top of the specification (#214).