Automatic train stations (#1353)

## Description:

Train stations are now built automatically when a factory is
constructed.

Changes:
- When a factory is built, nearby structures are connected to the rail
network
- When a city is built near a factory, it is connected to the rail
network
    - All structures behave the same when a train stops: to be defined
    - Removed station badge
    - Gold income is now related to the structure's level

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [x] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:

IngloriousTom

src/core/game/Game.ts

@@ -633,6 +633,12 @@ export interface Game extends GameMap {
   // Units
   units(...types: UnitType[]): Unit[];
   unitInfo(type: UnitType): UnitInfo;
+  hasUnitNearby(
+    tile: TileRef,
+    searchRange: number,
+    type: UnitType,
+    playerId: PlayerID,
+  );
   nearbyUnits(
     tile: TileRef,
     searchRange: number,