`webpack.dev.config.js` should be able to refer to use port number in `env.config.js` if provided

When we eventually move away from using .env configuration files, we'll want to ensure that the webpack.dev.config.js is able to provide the port number from env.config.js.

Currently, the port number for the devserver is determined here, which means that on npm run start it will only ever look at the process.env object for the PORT or it will default to 8080, which isn't ideal for developing with multiple repos running that don't have .env.* files anymore.

To fix this, we would need to figure out a way to bring the port number from env.config.js during runtime in webpack.dev.config.js or require all repos that now use env.config.js to modify the port assignment in their customized webpack dev config.

Example of an MFE with a .env.development AND env.config.js

// ./webpack.dev.config.js

const { createConfig } = require('@edx/frontend-build');

const envConfig = require('./env.config');

const config = createConfig('webpack-dev');

config.devServer.port = envConfig.PORT || process.env.PORT || 8080;

module.exports = config;

Start with config/webpack.dev.config.js and the port assignment referenced in the issue, then compare it with the env.config.js example and the npm run start flow. Determine how the dev server should obtain PORT at runtime and verify that the configured port is used instead of only process.env.PORT or 8080.