Metadata-only schema declaration support for use Scrypath.
Scrypath.Schema validates declaration options, stores normalized metadata on the
schema module, and exposes that metadata through __scrypath__/1.
Phase 1 supports these __scrypath__/1 keys:
:config:fields:filterable:sortable:settings:document_id:document_source:backend
The macro does not generate runtime APIs such as search/2 or reindex/1.