Scripts: Add lint-style script based on stylelint

[gziolo]

Description

This PR adds lint-style to wp-scripts exposed by @wordpress/scripts package.

It helps enforce coding style guidelines for your style files. It uses stylelint with the set of default rules provided. You can override them with your own rules as described in stylelint user guide.

How has this been tested?

npm run lint-css still works properly in the repository.
./node_modules/.bin/wp-scripts lint-style '**/*.scss' also works properly

[adamsilverstein]

Looks good to me!

[ntwb]

I think we should follow up in a subsequent pull request to add docs and a configuration for linting SCSS files.

---

I originally wrote the above and what I was going to write here was about using lint-style as the command instead of lint-css, expanding on this thinking, we would want to add lint-scss for the scss config and then have lint-style run both lint-css and lint-scss?

[ntwb]

The .stylelintrc.json → packages/scripts/config/.stylelintrc.json change copies the Gutenberg specific .stylelintrc.json stylelint configuration to be distributed by @wordpress/scripts

This file should remain in its current location and a new file should be created at packages/scripts/config/.stylelintrc.json that contains:

{
	"extends": "stylelint-config-wordpress"
}

The README.md should be updated to:

It uses [stylelint](https://github.com/stylelint/stylelint) with the [stylelint-config-wordpress](https://github.com/WordPress-Coding-Standards/stylelint-config-wordpress) configuration per the [WordPress CSS Coding Standards](https://make.wordpress.org/core/handbook/best-practices/coding-standards/css/).

[ntwb]

The notes at #12722 (comment) should be addressed before merge

[gziolo]

Thanks @ntwb for sharing your feedback. I'm in transit and will get back to it later this week. Unless you want to apply proposed changes yourself :)

[gziolo]

I originally wrote the above and what I was going to write here was about using lint-style as the command instead of lint-css, expanding on this thinking, we would want to add lint-scss for the scss config and then have lint-style run both lint-css and lint-scss?

What would be the difference? I assumed they are the same in the context of Gutenberg.

[gziolo]

@ntwb I applied all your suggestions in 668eebb.