fix(types): use undefined instead of void return type (#5)

In this context, the lookup function returning `undefined` is information - it is a union of Some(data) | None. Void should be used for functions that really return no data.
This has practical implications as in typescript, a property typed `string | undefined` cannot have `string | void` assigned to it.

index.d.ts

@@ -1,2 +1,2 @@
 export const mimes: Record<string, string>;
-export function lookup(extension: string): string | void;
+export function lookup(extension: string): string | undefined;