Add a CLI option for exporting a vanilla ESLint config

[jamestalmage]

See https://github.com/jamestalmage/xo-with-webstorm

Lots of editors support ESLint directly. Not that many support XO. We should add a xo --export-config command that outputs a .eslintrc file, and ensures all the correct packages are installed at the top level.

$ xo --export-config
This will overwrite the existing .eslintrc file. Ok? (y/n) y

Some of XO's modules need to be installed as top level dependencies for this to work.
Enter the following command:

   npm install --save-dev eslint-config-xo eslint-plugin... 

[sindresorhus]

Related to #122.

I'm ok with adding this flag.

[pvdlg]

XO has it own implementation of overrides which mean when we export the ESLint config we would have to translate from XO implementation to ESLint.

As it is now supported by ESLint, shouldn't we remove the XO override implementation and use ESLint one instead?

That would make this feature slightly easier to implement

[slikts]

Generating an ESLint config that would extend the XO config would make editor integrations work out-of-the-box for all the editors that support it; instead, now XO-specific editor extensions are required. I understand this is a legacy issue, but it's a significant letdown.

Edit: adding to it, the VSCode integration doesn't work for me at all.

[fregante]

The new Flat config format essentially covers these needs:

• If you want an all-in-one dependency, install and run xo
• If you want an eslint config, install eslint-config-xo, which will soon include all the necessary dependencies

Since this issue is now 5 years old with little progress/interest + Prettier and TypeScript having become dependencies (further complicating the endeavor), I'm closing in favor of

• Move to flat config #702