feat: only allow specific selectable values for the cooldown duration of spells

World data (including compendium packs) is migrated automatically. In order to also migrate packs
provided by modules, you can use the following macro:
```js
const pack = game.packs.get("<name-of-the-module>.<name-of-the-pack>");
game.ds4.migration.migrateCompendiumFromTo(pack, 4, 5);
```

src/migrations/002.ts

@@ -12,7 +12,7 @@ import {
     migrateScenes,
 } from "./migrationHelpers";
 
-export async function migrate(): Promise<void> {
+async function migrate(): Promise<void> {
     await migrateItems(getItemUpdateData);
     await migrateActors(getActorUpdateData);
     await migrateScenes(getSceneUpdateData);
@@ -32,3 +32,8 @@ const migrateCompendium = getCompendiumMigrator(
     { getItemUpdateData, getActorUpdateData, getSceneUpdateData },
     { migrateToTemplateEarly: false },
 );
+
+export const migration = {
+    migrate,
+    migrateCompendium,
+};