Default datatype values

[timothee-haudebourg]

This PR implement the concept of default values for datatypes. This relates to the default keyword found in JSON Schema or Lexicon.

• Define a tldr:defaultValue property.
• Add datatype default values in the core data model.
• Implement default values in the build data model.
• JSON Schema default keyword support (for datatypes).
• Lexicon default keyword support.
• Generate Rust code for default values.
• Testing