Add format-text-underline-squiggle (better name suggestions welcome!) icons as requested in https://bugs.kde.org/show_bug.cgi?id=408283 (not specifying the BUG keyword, since this doesn't add all requested icons).
The curve of the underline is not great, but the best I could come up with after many tries, and I made the "U" in the 16px version 2px less tall compared to the `format-text-underline` icon, not sure if that should be changed there, too.