Add `undefined` to optional properties in TypeScript definitions

For example, `slideOffsetBefore` property is defined as:

```ts
slidesOffsetBefore?: number;
```

When using the `tsconfig.json` option `exactOptionalPropertyTypes`, it becomes cumbersome to set the `slidesOffsetBefore` to `undefined`. Updating the TypeScript definitions like so would improve the developer experience:

```ts
slidesOffsetBefore?: number | undefined;
```