Stylelint: support css-modules syntax such as `:global`

[p2edwards]

Now that we use CSS Modules in kpi, we may accept css modules specific syntax.

Example:

// Allow :global pseudo-class (CSS Modules)
'selector-pseudo-class-no-unknown': [
  true,
  {ignorePseudoClasses: ['global', /* ... */]},
],

We could either:

• Extend 'stylelint-config-css-modules', either in kobo-common or in kpi after extending kobo-common.
• Or add rules a la carte. 'stylelint-config-css-modules' index.js is a good reference.