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/execution/CityExecution.ts

@@ -1,5 +1,6 @@
 import { Execution, Game, Player, Unit, UnitType } from "../game/Game";
 import { TileRef } from "../game/GameMap";
+import { TrainStationExecution } from "./TrainStationExecution";
 
 export class CityExecution implements Execution {
   private mg: Game;
@@ -24,6 +25,7 @@ export class CityExecution implements Execution {
         return;
       }
       this.city = this.player.buildUnit(UnitType.City, spawnTile, {});
+      this.createStation();
     }
     if (!this.city.isActive()) {
       this.active = false;
@@ -42,4 +44,18 @@ export class CityExecution implements Execution {
   activeDuringSpawnPhase(): boolean {
     return false;
   }
+
+  createStation(): void {
+    if (this.city !== null) {
+      const nearbyFactory = this.mg.hasUnitNearby(
+        this.city.tile()!,
+        this.mg.config().trainStationMaxRange(),
+        UnitType.Factory,
+        this.player.id(),
+      );
+      if (nearbyFactory) {
+        this.mg.addExecution(new TrainStationExecution(this.city));
+      }
+    }
+  }
 }