allow alliance extension Fixes #491 (#1314)

## Description:

About 30s before an alliance is about to expire, both players receive a
prompt to extend the alliance. If both players agree the alliance is
extended.

## 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/execution/alliance/AllianceExtensionExecution.ts

@@ -0,0 +1,61 @@
+import {
+  Execution,
+  Game,
+  MessageType,
+  Player,
+  PlayerID,
+} from "../../game/Game";
+
+export class AllianceExtensionExecution implements Execution {
+  constructor(
+    private readonly from: Player,
+    private readonly toID: PlayerID,
+  ) {}
+
+  init(mg: Game, ticks: number): void {
+    if (!mg.hasPlayer(this.toID)) {
+      console.warn(
+        `[AllianceExtensionExecution] Player ${this.toID} not found`,
+      );
+      return;
+    }
+    const to = mg.player(this.toID);
+    const alliance = this.from.allianceWith(to);
+    if (!alliance) {
+      console.warn(
+        `[AllianceExtensionExecution] No alliance to extend between ${this.from.id()} and ${this.toID}`,
+      );
+      return;
+    }
+
+    // Mark this player's intent to extend
+    alliance.addExtensionRequest(this.from);
+
+    if (alliance.canExtend()) {
+      alliance.extend();
+
+      mg.displayMessage(
+        "alliance.renewed",
+        MessageType.ALLIANCE_ACCEPTED,
+        this.from.id(),
+      );
+      mg.displayMessage(
+        "alliance.renewed",
+        MessageType.ALLIANCE_ACCEPTED,
+        this.toID,
+      );
+    }
+  }
+
+  tick(ticks: number): void {
+    // No-op
+  }
+
+  isActive(): boolean {
+    return false;
+  }
+
+  activeDuringSpawnPhase(): boolean {
+    return false;
+  }
+}