Lexicon support

[timothee-haudebourg]

Lexicon is a JSON-based schema definition framework.

It is basically JSON Schema wrapped around a Lexicon TypeScript-like interface that tells what the schema is used for (query, procedure, record, or token). There is no formal definition yet. The best I could find is a definition of some of the TypeScript types. It is missing the JSON Schema part they use to define constraints, it may not be fully compliant with JSON Schema.

The implementation will be very similar to the JSON Schema one. The only issue is with "tokens". To implement them we will need #137.