Add support for instant + parallel event function handlers

[notunderctrl]

The current event handler implementation calls event functions in an order, while waiting for the previous one to finish. Some situations require multiple, instant, and parallel handling which the current handler lacks.

Proposed structure

// File: src/app/events/interactionCreate/handler.js

// an export to treat this handler differently from the ones running in sequence
// ckit might use something like Promise.all() to call them all at once
export const parallel = true; // variable name might need work

export default async function (interaction) {
  // handle event
}