feat: add webhook event constructor

[carstenlebek]

I have added a webhook event constructor similar to stripe.

import { constructEvent } from "lemonsqueezy.ts";

const event = constructEvent(
    "RAW_REQUEST_BODY",
    "X-Signature HEADER_VALUE",
    "YOUR_WEBHOOK_SECRET"
)

switch (event.type) {
    case "order_created":
        const order = event.data;
        break;
    case "order_refunded":
        const order = event.data;
        break;
    case "subscription_created":
        const subscription = event.data;
        break;
    default:
        break;
}