build, msvc: Enable preprocessor conformance mode

See:
- https://learn.microsoft.com/en-us/cpp/build/reference/zc-preprocessor
- https://learn.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview

Otherwise, the "traditional" MSVC preprocessor fails to parse the
`FUZZ_TARGET` and `DETAIL_FUZZ` macros because of behavior changes
highlighted in the docs mentioned above.

build_msvc/common.init.vcxproj.in

@@ -87,7 +87,7 @@
     <ClCompile>
       <WarningLevel>Level3</WarningLevel>
       <PrecompiledHeader>NotUsing</PrecompiledHeader>
-      <AdditionalOptions>/utf-8 /Zc:__cplusplus /std:c++20 %(AdditionalOptions)</AdditionalOptions>
+      <AdditionalOptions>/utf-8 /Zc:preprocessor /Zc:__cplusplus /std:c++20 %(AdditionalOptions)</AdditionalOptions>
       <DisableSpecificWarnings>4018;4244;4267;4715;4805</DisableSpecificWarnings>
       <TreatWarningAsError>true</TreatWarningAsError>
       <PreprocessorDefinitions>_SILENCE_CXX17_CODECVT_HEADER_DEPRECATION_WARNING;SECP256K1_STATIC;ZMQ_STATIC;NOMINMAX;WIN32;HAVE_CONFIG_H;_CRT_SECURE_NO_WARNINGS;_CONSOLE;_WIN32_WINNT=0x0601;_WIN32_IE=0x0501;WIN32_LEAN_AND_MEAN;%(PreprocessorDefinitions)</PreprocessorDefinitions>