fix: adds default api key file

lib/src/api/api_keys.default.dart

@@ -0,0 +1,10 @@
+/// To get an API key, sign up here:
+/// https://home.openweathermap.org/users/sign_up
+///
+/// Then paste it in here and rename this file to `api_keys.dart`.
+///
+/// You can also specify an API Key via --dart-define. Example:
+/// "flutter run --dart-define API_KEY=YOUR_API_KEY
+class APIKeys {
+  static const String? openWeatherAPIKey = null;
+}