Vend a typeguard function for checking if an Error is a PurchasesError

Hey. When using TypeScript, the type of the caught error is unknown (any in non-strict mode of older versions of TypeScript). It would be very useful to have a typeguard function vended that could narrow the type. Would you be willing to accept a PR that implements this?

export const isPurchasesError = (err: unknown): err is PurchasesError => {
  // Implementation here
}

The usage would look like:

try {
  await someCodeThatTriggersAPurchase()
} catch (err) {
  if (isPurchasesError(err)) {
    // Handle PurchasesError here. 
    // The type will be narrowed by the compiler and IDE completion will work nicely.
  } else {
    // Handle other errors differently
  }
}

This is something we can of course just write on our own end, but it seems like it'd be useful to have as part of the core library here.

Start by locating the existing PurchasesError type and the package's public TypeScript exports. Define the exported guard so it narrows unknown errors to PurchasesError, then verify that the example catch block receives the narrowed type and that non-PurchasesError values are rejected.