Update webpack config to output UMD library

[fuchstim]

What's Changing and Why

This addresses an issue when importing Jimp in a browser environment (detailed issue with workarounds here: #1194):

import Jimp from 'jimp';
// Jimp = {}

const Jimp = require('jimp');
// Jimp = {}

// Workaround:
import type TJimp from 'jimp';
import 'jimp';

const { Jimp } = window as unknown as { Jimp: typeof TJimp };

The proposed fix correctly configures webpack to emit a library export, which both fixes the issue above as well as maintains compatibility with current import methods (and workarounds).