work with exactOptionalPropertyTypes

Need to use the FC type that is passed in (which can extend *either*
`void` *or* `undefined`, because those are OBVIOUSLY TWO COMPLETELY
DIFFERENT THINGS, according to TSC in its infinite wisdom, and this is
totally not something I'm needlessly and constantly salty about) rather
than setting it to `undefined` optionally, because heaven forbid someone
passes `void` in there, you might treat it as `undefined` instead of
`void`, which would be so unsafe and bad.

Anyway. Probably a completely normal amount of feelings to have about a
programming language wart, right?

fix: #406

Author: isaacs

src/index.ts

@@ -503,7 +503,7 @@ export namespace LRUCache {
     V,
     FC extends undefined | void = undefined,
   > extends FetchOptions<K, V, FC> {
-    context?: undefined
+    context?: FC
   }
 
   export interface MemoOptions<K, V, FC = unknown> extends Pick<
@@ -556,7 +556,7 @@ export namespace LRUCache {
     V,
     FC extends undefined | void = undefined,
   > extends MemoOptions<K, V, FC> {
-    context?: undefined
+    context?: FC
   }
 
   /**