Add linting for styles

[thibaudcolas]

Chore. Add linting of styles with https://github.com/stylelint/stylelint.

[thibaudcolas]

stylelint property in package.json https://github.com/stylelint/stylelint/blob/master/docs/user-guide/configuration.md, extending:

• https://github.com/stylelint/stylelint-config-standard
• https://github.com/kristerkari/stylelint-config-recommended-scss
• https://github.com/prettier/stylelint-config-prettier

Potentially extract the config to a separate package.