Create babel preset instead of copying entire config into project

[skipkayhil]

Currently, webpacker copies the pre made babel.config.js into your project when the install script is run, however this leads to some issues:

• users who would never touch their babel.config.js now have a complicated file they have to maintain
• updates to babel.config.js in webpacker repo have to be manually grabbed by users
• the babel.config.js in the project uses require on webpacker dependencies, which is not ideal and is a blocker to adopting Yarn 2 Yarn 2.0 (Yarn Plug’n’Play) #2112

To solve these issues, I think webpacker (or an accompanying package) should provide a babel preset that projects can then use:

// babel.config.js
module.exports = {
  presets: ["@rails/babel-preset-webpacker"]
}

or

// babel.config.js
module.exports = {
  presets: ["./node_modules/@rails/webpacker/.../babel.config.js"]
}

This would greatly simplify the configuration for basic cases, while still allowing the flexibility to add additional configuration or ignoring the preset completely for advanced cases.

I am willing to help out with this and can create a PR if needed.

[skipkayhil]

@gauravtiwari You've mentioned this before, how can I help to get this done?