icu: Add a stub DLL.
These two functions are enough to get several games running (ScarQuest, Marvel Snap).
In the long run I'm not sure how we want to proceed with this DLL. ICU has a complicated license and is largely C++, so integrating it into wine under libs/
may be tricky. They do distribute official Windows binaries, and Proton is repackaging those and adding an icu.dll that forwards to them.