fix: remove webpack-preprocessor from cypress config

Removes the `@cypress/webpack-preprocessor` from the generated cypress configuration, as it leads to several issues regarding file watching, headless mode and TS support.
Note that the dependency is still there, so any user needng it back just have to uncomment the lines in the generated config.

Fixes #2903

Author: cexbrayat

packages/@vue/cli-plugin-e2e-cypress/generator/template/tests/e2e/plugins/index.js

@@ -1,12 +1,18 @@
 // https://docs.cypress.io/guides/guides/plugins-guide.html
-/* eslint-disable import/no-extraneous-dependencies, global-require */
-const webpack = require('@cypress/webpack-preprocessor')
+
+// if you need a custom webpack configuration you can uncomment the following import
+// and then use the `file:preprocessor` event
+// as explained in the cypress docs
+// https://docs.cypress.io/api/plugins/preprocessors-api.html#Examples
+
+// /* eslint-disable import/no-extraneous-dependencies, global-require */
+// const webpack = require('@cypress/webpack-preprocessor')
 
 module.exports = (on, config) => {
-  on('file:preprocessor', webpack({
-    webpackOptions: require('@vue/cli-service/webpack.config'),
-    watchOptions: {}
-  }))
+  // on('file:preprocessor', webpack({
+  //  webpackOptions: require('@vue/cli-service/webpack.config'),
+  //  watchOptions: {}
+  // }))
 
   return Object.assign({}, config, {
     fixturesFolder: 'tests/e2e/fixtures',