Add property validators
Compatible with SonarQube 5.6+
Release notes: https://github.com/racodond/sonar-css-plugin/milestone/4?closed=1
- Accept 8 digit color and 4 digit color
- Add properties 'place-content', 'place-self', 'place-items'
- Add validator for properties 'align-self', 'justify-self', 'align-items', 'justify-items'
- Add validator for properties 'text-wrap', 'wrap-after' and 'wrap-before'
- Add validator for properties 'border-image-outset', 'border-image-repeat', 'border-image-width'
- Add validator for property 'will-change'
- Update validators for properties 'justify-content' and 'align-content'
- Fix validator for property 'background-position-y'