[D8] Trim summary on word boundary

As per https://github.com/backdrop/backdrop-issues/issues/309, can we also add in this functionality: Field Word Boundary? This module allows trimming to word boundaries, not just sentences or paragraphs. Seems like a logical addition to core...

(I think the problem this solves is most often seen when the trim length is quite short (e.g. 100 characters) and the first sentence is longer than this - this means that nothing gets output).