feat: redesign consumed REST client syntax and add full mapping support

Redesigns CREATE REST CLIENT to use property-based {} syntax aligned
with CREATE PUBLISHED REST SERVICE:

  CREATE REST CLIENT Module.Api (
    BaseUrl: 'https://...',
    Authentication: NONE
  )
  {
    OPERATION GetItems {
      Method: GET,
      Path: '/items/{id}',
      Parameters: ($id: String),
      Query: ($q: String),
      Headers: ('Accept' = 'application/json'),
      Response: MAPPING Module.ItemResponse {
        "Id" = "id",
      }
    }
  };

New capabilities:
- Body: TEMPLATE '...' (Rest$StringBody with value template)
- Body: MAPPING Entity { jsonField = Attr } (Rest$ImplicitMappingBody)
- Response: MAPPING Entity { Attr = jsonField } (Rest$ImplicitMappingResponseHandling)
- Nested object mappings: CREATE Assoc/Entity = jsonField { ... }
- JsonPath comments in DESCRIBE for tracing JSON flattening
- Query param types now parsed correctly (was hardcoded to String)
- DESCRIBE always double-quotes mapping identifiers for roundtrip safety
- Version feature keys and checkFeature() for published REST commands
- Documentation: quick reference, help topics, docs-site examples

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako