chore(config): add stylelint for CSS code quality (#4524)

- Add stylelint with stylelint-config-standard and stylelint-config-clean-order
    - Configure custom rules for CSS custom properties (kebab-case/camelCase with pseudo-private support)
    - Enforce BEM class naming with eds- prefix
    - Add comment spacing rules
    - Configure vendor prefix exceptions for column properties
    - Add lint:css script targeting /next components
    - Include stylelint VS Code extension in workspace recommendations

Author: vnys

.stylelintrc.yaml

@@ -0,0 +1,17 @@
+extends:
+  - 'stylelint-config-standard'
+  - 'stylelint-config-clean-order'
+rules:
+  custom-property-pattern:
+    - '^_?([a-z][a-zA-Z0-9]*)(-[a-zA-Z0-9]+)*$'
+    - message: "Expected '%s' to be kebab-case or camelCase, optionally prefixed with _ for pseudo-private properties (custom-property-pattern)"
+  property-no-vendor-prefix:
+    - true
+    - ignoreProperties: ['/^(column[s-]?)([a-z]*)$/']
+  comment-empty-line-before:
+    - always
+    - except: [first-nested]
+      ignore: [after-comment, stylelint-commands]
+  selector-class-pattern:
+    - '^eds-[a-z][a-z0-9]*(-[a-z0-9]+)*(__[a-z][a-z0-9]*(-[a-z0-9]+)*)?(--[a-z][a-z0-9]*(-[a-z0-9]+)*)?$'
+    - message: "Expected class '%s' to follow BEM syntax: eds-block__element--modifier (selector-class-pattern)"

package.json

@@ -9,6 +9,7 @@
     "lint:data-grid-react": "pnpm run lint ./packages/eds-data-grid-react",
     "lint:lab": "pnpm run lint ./packages/eds-lab-react",
     "lint:utils": "pnpm run lint ./packages/eds-utils",
+    "lint:css": "stylelint ./packages/eds-core-react/src/components/next/**/*.css",
     "init": "pnpm install --force && pnpm run build",
     "extract-prerequisites": "node scripts/extract-prerequisites.js",
     "prebuild": "pnpm run extract-prerequisites",
@@ -88,6 +89,10 @@
     "eslint-plugin-react-hooks": "^5.2.0",
     "eslint-plugin-testing-library": "^7.15.4",
     "jest-axe": "^10.0.0",
+    "stylelint": "^17.3.0",
+    "stylelint-config-clean-order": "^8.0.1",
+    "stylelint-config-standard": "^40.0.0",
+    "stylelint-order": "^7.0.1",
     "prettier": "3.8.1",
     "typescript": "^5.9.3"
   },