fix: dont require index signature in event map typing

Without this change, the following error appears when using an interface
of event listeners:

Type 'MyEventListeners' does not satisfy the constraint 'ValidEventTypes'.
  Type 'MyEventListeners' is not assignable to type '{ [x: string]: any[] | ((...args: any[]) => void); }'.
    Index signature is missing in type 'NodeRunnerEventListeners<Output>

index.d.ts

@@ -94,7 +94,7 @@ declare namespace EventEmitter {
   export type ValidEventTypes =
     | string
     | symbol
-    | { [K in string | symbol]: any[] | ((...args: any[]) => void) };
+    | { [K in string | symbol]: any[] | ((...args: any[]) => void) | undefined };
 
   export type EventNames<T extends ValidEventTypes> = T extends string | symbol
     ? T