Increase trade ship spawn rate (#1455)

## Description:

During play testing not enough trade ships were spawning in the mid-late
game. This increases their spawn rate and max number of ships.

## 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:

evan

src/core/configuration/DefaultConfig.ts

@@ -356,17 +356,14 @@ export class DefaultConfig implements Config {
 
   // Chance to spawn a trade ship in one second,
   tradeShipSpawnRate(numTradeShips: number): number {
-    if (numTradeShips <= 20) {
+    if (numTradeShips < 20) {
       return 5;
     }
-    if (numTradeShips > this.tradeShipCap()) {
-      return 1_000_000;
+    if (numTradeShips <= 150) {
+      const additional = numTradeShips - 20;
+      return Math.pow(additional, 0.8) + 5;
     }
-    return numTradeShips - 15;
-  }
-
-  tradeShipCap(): number {
-    return 100;
+    return 1_000_000;
   }
 
   unitInfo(type: UnitType): UnitInfo {