Infers the merged extensions type from a tuple of transformer factory functions.
Given [() => StreamTransformer<{ a: number }>, () => StreamTransformer<{ b: string }>],
produces { a: number } & { b: string }.
InferExtensions: T extends readonly [] Record<
string,
never
> : T extends readonly [(
) => StreamTransformer<P>, ...Rest] P InferExtensions<Rest> : Record<string, unknown>