Ability to control where line breaks appear

[valadaptive]

That is, something like the word-break CSS property. Right now, Parley breaks all lines at word boundaries (there's logic to insert "emergency line breaks" if a single word overflows the maximum width, but it seems to be disabled).

For egui, we need to be able to customize where line breaks can appear (strictly at word boundaries, at word boundaries but falling back to letter boundaries, or at letter boundaries).