This change adds generic variants of GeoJSON features and feature collections so callers can use typed property structs instead of the default map[string]interface{}.

It introduces FeatureOf[P] and FeatureCollectionOf[P].

This preserves all existing behavior

AI summary below.

What Changed

• Added FeatureOf[P] and FeatureCollectionOf[P]
• Kept Feature and FeatureCollection as aliases for backward compatibility
• Updated JSON/BSON marshal and unmarshal logic to work with generic property types
• Added tests covering generic JSON and BSON round-trips
• Documented typed property usage in README.md and geojson/README.md

Why

• Lets consumers deserialize GeoJSON into strongly typed property structs
• Avoids repeated manual casting from map[string]interface{}
• Preserves the existing default behavior for users who do not need typed properties

Tests

• Added feature JSON round-trip coverage for typed properties
• Added feature BSON round-trip coverage for typed properties
• Added feature collection JSON round-trip coverage for typed properties