This requires TypeScript users to explicitly define the type of the data they
are consuming.

Before this, data was `any` by default. This means TypeScript consumers didn’t
get type safety if they forgot to specify the type.

Technically this is a breaking change for TypeScript users, as this will report
errors if they forgot to specifiy the response type. The simplest workaround
would be to explicitly set the response type to `any`, so it’s not breaking
much.

The `unknown` type is probably a slightly better fit, but this requires
TypeScript ^3.

`data` is still `any` in the very specific use case mentioned in
microsoft/TypeScript#38969