LibUnicode: Move Unicode-aware case transformations to a helper file

These will be needed by AK::String as well, so move them to a helper
file where they can be re-used.

Userland/Libraries/LibUnicode/CMakeLists.txt

@@ -5,6 +5,7 @@ set(SOURCES
     CurrencyCode.cpp
     Emoji.cpp
     Normalize.cpp
+    UnicodeUtils.cpp
     ${UNICODE_DATA_SOURCES}
 )
 set(GENERATED_SOURCES ${CURRENT_LIB_GENERATED})