Remove webpack imports

[milan338]

In Next.js, webpack itself is bundled together with Next, and through this, plugins used appear to be unable to require webpack from node_modules. However, webpack 5 exposes compiler.webpack to plugins, meaning webpack imports can be circumvented. This PR does not change any logic, rather it just utilises this new hook that enables usage with Next. An example Next.js config:

const ThreeMinifierPlugin = require('@yushijinhun/three-minifier-webpack');

module.exports = {
    webpack: (config, { isServer, dev }) => {
        if (!isServer && !dev) {
            config.cache = false;
            const threeMinifier = new ThreeMinifierPlugin();
            config.plugins.unshift(threeMinifier);
            config.resolve.plugins.unshift(threeMinifier.resolver);
        }
        return config;
    },
};