fix(geocoding_darwin): declare pigeon as a dev_dependency

[bjrochem72]

Moves pigeon from dependencies to dev_dependencies in geocoding_darwin.

Pigeon is a build-time code generator. The generated lib/src/geocoding/geocoding.g.dart is self-contained and doesn't import package:pigeon at runtime, so it doesn't need to be a runtime dependency. geocoding_android already declares it under dev_dependencies; this just brings geocoding_darwin in line.

Because it's a regular dependency today, pigeon is pulled into every app that depends on geocoding, and pigeon's analyzer >=10.0.0 <13.0.0 constraint then caps the whole app's analyzer at 12. That blocks any package in the app that needs analyzer 13+. In our project it prevents upgrading riverpod_generator/riverpod_lint (3.4+) and drift_dev, which all require analyzer ^13, and there's no workaround short of dropping geocoding since it's the only path pulling pigeon in.

Fixes #304.

Verification:

• grep -rn "package:pigeon" geocoding_darwin/lib returns nothing (no runtime use).
• flutter pub get in the example app still resolves.