types: index is no longer optional for the estimateSize type

Fixes #24

Author: tannerlinsley

types/index.d.ts

@@ -18,7 +18,7 @@ export type VirtualItem = {
 declare function useVirtual<T>(options: {
   size: number
   parentRef: React.RefObject<T>
-  estimateSize?: (index?: number) => number
+  estimateSize?: (index: number) => number
   overscan?: number
   horizontal?: boolean
   scrollToFn?: (