Add format-text-underline-squiggle actions icons

Summary:
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.

CCBUG: 408283

Test Plan:
The new format-text-underline-squiggle icons:

For reference, the format-text-underline icons:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ndavis, ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D24185