json-schema new drafts

Fixing some small bugs:

• additional properties default is true
• fill properties with required properties
• possible infered types with type fields

Some optional improvements:

• Add local _opam to .gitignore
• optionally add the dialect

Add new json-schema drafts:

• draft-06: reference: https://json-schema.org/draft-06/json-schema-release-notes changes:

  • $id instead of id
  • exclusiveMinimum and exclusiveMaximum as numbers
  • propertyNames for arrays
  • contains for arrays
  • const as single enum
  • examples

• draft-07: reference: https://json-schema.org/draft-07/json-schema-release-notes changes:

  • readOnly, writeOnly
  • if, then, else
  • contentMediaType, contentEncoding for strings

• draft-2019-09: reference: https://json-schema.org/draft/2019-09/release-notes changes:

  • minContains, maxContains

• draft-2020-12: reference: https://json-schema.org/draft/2020-12/release-notes changes:

  • structure with items, additionalItems and prefixItems for arrays