Unexpected word boundary with WB4 and WB6/WB7 rules

[anba]

Word segmentation over the string "a.\xADb" returns unexpected word breaks: The returned word segments are "a", ".\xAD", and "b". But I think WB4 should have prevented the word break, right? Compared to that, "a.b" and "a\xAD.b" are both correctly classified as a single word.

CC: @aethanyc and @makotokato

[sffc]

@eggrobin

[Manishearth]

#4550 fixes some of the issues here but not all of them, see #4550 (comment)

[robertbastian]

Tracking remaining changes in #5015