Slugify a string

Route segments, filenames, anchor IDs and CMS keys all need the same thing: an ASCII-safe string with no surprises. Paste any input and get one. Non-Latin scripts are transliterated rather than dropped, so a Cyrillic title produces a meaningful identifier instead of an empty string — which matters when the alternative is a route that silently collapses to nothing. Accents fold, everything else becomes your chosen separator, and runs collapse.