Fix typing error in loader function

Fixes #111

Update the `loader` function in `index.d.ts` to accept either a string `mcVersion` or a `bot.registry` object.

* Modify the TypeScript definition to reflect the possible `bot.registry` parameter.

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index.d.ts

@@ -40,4 +40,4 @@ export class Item {
   static nextStackId(): number;
 }
 
-export default function loader(mcVersion: string): typeof Item;
+export default function loader(mcVersion: string | object): typeof Item;