test: Migrate Polygon specs to the REST client

Ports spec/ParsePolygon.spec.js to REST-based specs under spec/rest/objects/:
- polygon.spec.ts: object lifecycle, equalTo, and validation
- polygon-query.spec.ts: $geoIntersects point-in-polygon queries (#4608)
- polygon-mongo.spec.ts: MongoDB storage format and 2d/2dsphere indexes

Adds polygon/geoIntersects builders to spec/helpers/geo.ts and a
describe_only_db ambient declaration. No Parse JS SDK; the storage spec uses
the database adapter directly only where there is no REST surface.

Author: dblythy