Gtk.TextIter.prototype.backward_sentence_start

function backward_sentence_start():Boolean {
    // Gjs wrapper for gtk_text_iter_backward_sentence_start()
}

Moves backward to the previous sentence start; if iter is already at the start of a sentence, moves backward to the next one. Sentence boundaries are determined by Pango and should be correct for nearly any language (if not, the correct fix would be to the Pango text boundary algorithms).