Addition of a document type definition for library configurations

A document type definition can help to work with corresponding files because it describes the involved data structures to some degree. [Such a DTD](https://en.wikipedia.org/wiki/Document_type_definition "Description for document type definitions") can be added so that further improvements will become easier for the safe data exchange around library configurations for static source code analysis.

A DTD format was standardised as a basis for XML documents. Some technical challenges and limitations were noticed so that [another standard like W3C XML Schema](https://en.wikipedia.org/wiki/XML_Schema_%28W3C%29 "Description for the XML Schema standard") evolved. Such a XML schema definition can also be added so that corresponding file handling might become more consistent.
* Would you like to try this approach out?
* How are the chances to integrate this update suggestion into your source code repository?