add(key, value) {
		if (this.has(key)) {
			this.get(key).push(value);
		} else {
			this.set(key, [value]);
		}
	}
Adds the `value` to the `key`'s array of values.
@param {*} key Key to set.
@param {*} value Value to add to `key`'s array.
@returns {undefined} undefined.