Fix checkItemAvailability typings

[MrBartusek]

/check-availability endpoint returns success and items rather than success and data

Temporary fix:

const result = await this.waxpeer.checkItemAvailability([/* ... */])
// @ts-expect-error invalid waxpeer library typings
const items = result.items as Awaited<ReturnType<Waxpeer['checkItemAvailability']>>['data']