Lexicon support

[timothee-haudebourg]

This PR adds supports for AT Protocol's Lexicon schema definition format. It focuses on importing Lexicon documents as TreeLDR layouts.

This will be very similar to the JSON Schema implementation (hopefully better than that since I learned a lot since I implemented JSON Schemas).

Implementation plan

• Create the treeldr-lexicon library
• Define the core Lexicon data model
• Lexicon document deserialization
• Update the load library to recognize lexicon files
  • Refactor the library to avoid conflict with JSON Schema (which appens because it is also a JSON-based format)
  • Refactor treeldr-json-schema to use json-syntax and not serde-json, to be compatible with treeldr-lexicon and have better error handling
• Non-media type
• Media types: Image, Video, Audio
• Tokens support
• Type constraints (the ones supported by TreeLDR at least)
• Testing

[timothee-haudebourg]

It turns out the specification on AT Protocol's website is severely outdated. I now use their source code to infer the correct Lexicon interfaces specification, which is more complicated than the one presented on their website.

[timothee-haudebourg]

I have found a Lexicon schema that does not satisfies the specification given in the source code. According to the spec here the main type must be either record, procedure, query or subscription. In the given schema it is object.

I need to investigate to see if the source code specification is outdated, or if this schema is no longer used.

I think I misunderstood. The main definition is not necessarily a record, procedure, query or subscription, but any such type must be the main definition.

[timothee-haudebourg]

The Lexicon spec defines media types (Image, Video, Audio), but those types are never allowed in a Lexicon schema. I'm not sure of their purpose yet.