build: skip linkLibCpp on MSVC targets

Zig's bundled libc++/libc++abi conflicts with the MSVC C++ runtime
headers (vcruntime_typeinfo.h, vcruntime_exception.h, etc.) when
targeting native-native-msvc. This caused compilation failures in
the SIMD C++ code due to -nostdinc++ suppressing MSVC headers and
libc++ types clashing with MSVC runtime types.

Skip linkLibCpp() for MSVC targets across all packages (highway,
simdutf, utfcpp) and the main build (SharedDeps, GhosttyZig) since
MSVC provides its own C++ standard library natively. Also add
missing <iterator> and <cstddef> includes that were previously
pulled in transitively through libc++ headers but are not
guaranteed by MSVC's headers.

src/build/SharedDeps.zig

@@ -399,8 +399,12 @@ pub fn add(
         step.addIncludePath(b.path("src/apprt/gtk"));
     }
 
-    // libcpp is required for various dependencies
-    step.linkLibCpp();
+    // libcpp is required for various dependencies. On MSVC, the C++
+    // standard library is provided by the MSVC runtime and linking
+    // libc++ would conflict with it.
+    if (step.rootModuleTarget().abi != .msvc) {
+        step.linkLibCpp();
+    }
 
     // We always require the system SDK so that our system headers are available.
     // This makes things like `os/log.h` available for cross-compiling.