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/simd/codepoint_width.cpp

@@ -6,6 +6,7 @@
 #include <hwy/print-inl.h>
 
 #include <cassert>
+#include <iterator>
 
 HWY_BEFORE_NAMESPACE();
 namespace ghostty {

src/simd/index_of.h

@@ -6,6 +6,8 @@
 #endif
 
 #include <hwy/highway.h>
+
+#include <cstddef>
 #include <optional>
 
 HWY_BEFORE_NAMESPACE();