FindICU: Adapt to pick up custom ICU from Windows SDK

Modern Windows versions ship a subset of ICU:

https://learn.microsoft.com/en-us/windows/win32/intl/international-components-for-unicode--icu-

Unfortunately, this version of ICU is source compatible from a 'standard' ICU build, as it expects a custom include header and import lib (icu.h, icu.lib). This means that the standard FindICU.cmake won't detect it.

Using the built-in ICU capabilities is arguably very useful though, especially for cross-platform apps. Could FindICU.cmake be adapted to handle the Windows ICU transparently?