Adding multiple item boxes support

[ThembaMahlangu]

Itembox notifications can show in a row of up to 5 notifications at once

Normally if it receives 2 or more removal or add events of different items it will only show 1 item box notification instead of all

Checklist

• I have personally loaded this code into an updated qbcore project and checked all of its functionality.
• My code fits the style guidelines.
• My PR fits the contribution guidelines.

[0xF3546]

There's a thing called "DRY" in Programming (Don't repeat yourself).
I would recomment using it like this:

const notification = this.notifications[existingIndex];
notification.amount += .....
/*ALL THE STUFF */
this.notifications[existingIndex] = notification

[0xF3546]

You get the object 2 times, but you could just get it one time by assigning it to an object:

const timeOutId = this.notifications[index].timeoutId;
if (timeOutId) {
   clearTimeout(timeOutId)
}

[ThembaMahlangu]

Thanks for the review and recommendation, I'll look out for that more often

I also updated per recommendation

[0xF3546]

You forgit to update the object in notifications-array. JavaScript does not provide pointer's, so you have to update the object in the array again.
like this.notifications[existingIndex] = existingNotification