Add support for pseudo-elements and pseudo-classes

[Boldewyn]

There is postcss/postcss-selector-matches that acts on :matches() selectors. It's much too sophisticated for what is needed here, but might be a good starting point. It might be a bit tricky to correctly match the ones with parentheses like :not(). Suggested translations:

// pseudo-classes
':active': ':aktiv',
':checked': ':markiert',
':default': ':vorgabe',
':dir()': ':richtung()',
':disabled': ':gesperrt',
':empty': ':leer',
':enabled': ':aktiv',
':first': ':erstes',
':first-child': ':erstes-kind',
':first-of-type': ':erstes-vom-typ',
':fullscreen': ':vollbild',
':focus': ':fokus',
':hover': ':schweben',
':indeterminate': ':unbestimmt',
':in-range': ':im-bereich',
':invalid': ':ungültig',
':lang()': ':sprache()',
':last-child': ':letztes-kind',
':last-of-type': ':letztes-vom-typ',
':left': ':links',
':link': ':verknüpfung',
':not()': ':nicht()',
':nth-child()': ':ntes-kind()',
':nth-last-child()': ':n-letztes-kind()',
':nth-last-of-type()': ':n-letztes-vom-typ()',
':nth-of-type()': ':ntes-vom-typ()',
':only-child': ':einziges-kind',
':only-of-type': ':einziges-vom-typ',
':optional': ':optional',
':out-of-range': ':aus-dem-bereich',
':read-only': ':nur-lesen',
':read-write': ':lesen-schreiben',
':required': ':benötigt',
':right': ':rechts',
':root': ':wurzel',
':scope': ':umfang',
':target': ':ziel',
':valid': ':gültig',
':visited': ':besucht',

// pseudo-elements
'::after': '::danach',
'::before': '::davor',
'::first-letter': '::erster-buchstabe',
'::first-line': '::erste-zeile',
'::selection': '::auswahl',
':after': ':danach',
':before': ':davor',
':first-letter': ':erster-buchstabe',
':first-line': ':erste-zeile',
':selection': ':auswahl',

[timche]

That might be interesting. I would suggest to translate the element selectors as well, when it's possible.