[BUG] `this` context not available in typescript definitions

## Expected Behavior
inside a batch load function, `this` should refer to the data loader instance

## Current Behavior
`this` is not typed inside the batch load function

## Possible Solution

## Steps to Reproduce
attempt to use `this` in a batch loader function, see that typescript complains

## Context
my api call gives me back excess properties, so i want to prime my data loader with the excess data inside my batch function.