A simple addition to (weak) map

Personally I hate it to check if something is in a map, and if not add an object. So, I always write this small snippet,
I decided to make a repo out of it.

A simple example;

``typescript
import {WeakMapExt} from "map-ext";

type EventHandler = (...[]) => any;

const eventStore = new WeakMapExt>>(() => new MapExt(() => new Set()));

class Events {
async emit(key: string, ...args: any[]) {
for (const cb of eventStore.get(this).get(key)) {
await cb(...args);
}
}

on(key: string, cb: EventHandler) {
eventStore.get(this).get(key).add(cb);
}

off(key: string, cb: EventHandler) {
eventStore.get(this).get(key).delete(cb);
}
}
``

esm - commonjs

Since this package is so small it supports both esm and commonjs