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

[ryanrhee]

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.