Migrate to ES6 modules

Configure project to use ES6 modules to enable top-level await
capabilities. This change helps project to align well with modern JS
standards.

- Set `type` to `module` in `package.json`.
- Use import/export syntax in Cypress configuration files.
- Rename configurations files that do not support modules to use
  the `.cjs` extension:
  - `vue.config.js` to `vue.config.cjs` (vuejs/vue-cli#4477).
  - `babel.config.js` to `babel.config.cjs (babel/babel-loader#894)
  - `.eslintrc.js` to `.eslintrc.cjs` (eslint/eslint#13440,
    eslint/eslint#14137)
  - `postcss.config.js` to `postcss.config.cjs` (postcss/postcss#1771)
- Provide a workaround for Vue CLI & Mocha ES6 modules conflict in
  Vue configuration file (vuejs/vue-cli#7417).

tests/e2e/plugins/index.js

@@ -9,7 +9,7 @@
 // /* eslint-disable import/no-extraneous-dependencies, global-require */
 // const webpack = require('@cypress/webpack-preprocessor')
 
-module.exports = (on, config) => {
+export default (on, config) => {
   // on('file:preprocessor', webpack({
   //  webpackOptions: require('@vue/cli-service/webpack.config'),
   //  watchOptions: {}