Enable CRA-like environment variable passing in preact-cli

preact.config.js

import webpack from 'webpack'

export default {
    /**
     * Function that mutates the original webpack config.
     * Supports asynchronous changes when a promise is returned (or it's an async function).
     *
     * @param {object} config - original webpack config.
     * @param {object} env - options passed to the CLI.
     * @param {WebpackConfigHelpers} helpers - object with useful helpers for working with the webpack config.
     * @param {object} options - this is mainly relevant for plugins (will always be empty in the config), default to an empty object
     **/
    webpack(config, env, _helpers, _options) {
        config.resolve.alias['react'] = 'preact/compat'
        config.resolve.alias['react-dom'] = 'preact/compat'
        config.resolve.alias['@types/react'] = 'preact/compat'
        config.resolve.alias['@types/react-dom'] = 'preact/compat'

        config.plugins.push(new webpack.DefinePlugin(getClientEnvironment().stringified))
    },
}

// Get client environment, ala CRA. Use the same `REACT_APP_` prefix
// based on https://github.com/facebook/create-react-app/blob/5bd6e73047ef0ccd2f31616255c79a939d6402c4/packages/react-scripts/config/env.js

const REACT_APP = /^REACT_APP_/i
function getClientEnvironment() {
    const raw = Object.keys(process.env)
        .filter((key) => REACT_APP.test(key))
        .reduce((env, key) => {
            env[key] = process.env[key]
            return env
        }, {})

    const stringified = {
        'process.env': Object.keys(raw).reduce((env, key) => {
            env[key] = JSON.stringify(raw[key])
            return env
        }, {}),
    }

    return { stringified }
}