Add Nominatim Location Lexicon

[kayrozen]

This PR introduces a new location lexicon for the Nominatim geocoding service, enabling ATProto applications to reference locations from OpenStreetMap data.

What is Nominatim?
Nominatim is the geocoding service powering OpenStreetMap's search functionality. It provides comprehensive location data including addresses, points of interest, and administrative boundaries from the collaborative OpenStreetMap database.

Lexicon Design
The lexicon follows the established community pattern while adapting to Nominatim's data model:

• Required identifiers: Uses osm_id and osm_type for stable, permanent location references
• Core location data: Includes coordinates, display name, and location name
• Simplified structure: Focuses on essential fields for a v1 implementation

Benefits

• Open data: Built on OpenStreetMap's freely available, community-maintained dataset
• Global coverage: Comprehensive worldwide location database
• Stable references: OSM IDs provide permanent identifiers that don't change over time
• Extensible: Simple foundation that can be expanded with additional Nominatim fields in future versions

Example Usage

json{
  "osm_id": "1234567",
  "osm_type": "node",
  "latitude": "45.4215",
  "longitude": "-75.6972",
  "display_name": "Parliament Hill, Ottawa, Ontario, Canada",
  "name": "Parliament Hill"
}

This lexicon complements the existing Foursquare location lexicon by providing an open-source alternative for location referencing in the ATProto ecosystem.

[schuyler]

I suggest that we hold off on merging this, pending discussion within the ATGeo working group about how to represent places generally. I think we can come up with a generic lexicon that includes support for Nominatim records, etc.

[jphastings]

@schuyler , have there been any discussions? (Should I start them?)