Tell MSVC that our source files are UTF-8 encoded
For GCC/Clang that is the default, but not for MSVC. This can cause both compile-time and runtime errors when encountering string literals that contain more than just 7bit ASCII.
Some of our modules set this already, others use various other workarounds to avoid Utf-8 literals, so better do this consistently and centrally. Qt6 will also set this by default.
This is actually Hannah's finding from debugging issues caused by this in kitinerary, I'm just submitting the patch.