fix: should generate single text node for pseudo elements if possible
fix: should generate single text node for pseudo elements if possible

- fix #863

Unnecesarily fragmented text nodes are problematic for text-combine-upright properly working, so we should generate single text node as much as possible.

vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js,vivliostyle/vivliostyle.js