reduce train gold after each city (#3400)

## Description:

Now that cities snap to existing rails, it's possible to line up dozens
of cities in a row, producing way too much gold. This PR reduces the
gold after each stop to prevent that. Gold only starts decreasing after
the 3rd city.

## 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

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

evan

tests/NationStructureBehavior.test.ts

@@ -28,7 +28,7 @@ function makeStation(unit: any, cluster: Cluster | null = null): any {
 function makeGame(stations: any[] = []): any {
   return {
     config: () => ({
-      trainGold: (rel: string) => TRAIN_GOLD[rel] ?? 0n,
+      trainGold: (rel: string, _citiesVisited: number) => TRAIN_GOLD[rel] ?? 0n,
     }),
     railNetwork: () => ({
       stationManager: () => ({ getAll: () => new Set(stations) }),