Preset option for errors & warnings only

[bring-shrubbery]

I've noticed that I often need just see warnings and errors since during rapid development I don't really need to see the modules stats. So I tend to write the same code every time and it would be nice to be able to just provide an "errors-warnings" preset to save space and improve readability.

What kind of change does this PR introduce?

Extension to the existing stats preset.

Did you add tests for your changes?

Tests added.

Does this PR introduce a breaking change?

No, this is a just an extension which will not break anything

What needs to be documented once your changes are merged?

The following page will need to be updated with additional stats preset option: https://webpack.js.org/configuration/stats/

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[webpack-bot]

For maintainers only:

• This need to be documented (issue in webpack/webpack.js.org will be filed when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[webpack-bot]

The minimum test ratio has been reached. Thanks!

[sokra]

Thanks

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.