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.

pkg/highway/build.zig

@@ -20,7 +20,11 @@ pub fn build(b: *std.Build) !void {
         }),
         .linkage = .static,
     });
-    lib.linkLibCpp();
+    // On MSVC, the C++ standard library is provided by the MSVC runtime
+    // and linking Zig's bundled libc++ would conflict with it.
+    if (target.result.abi != .msvc) {
+        lib.linkLibCpp();
+    }
     if (upstream_) |upstream| {
         lib.addIncludePath(upstream.path(""));
         module.addIncludePath(upstream.path(""));

pkg/simdutf/build.zig

@@ -12,7 +12,11 @@ pub fn build(b: *std.Build) !void {
         }),
         .linkage = .static,
     });
-    lib.linkLibCpp();
+    // On MSVC, the C++ standard library is provided by the MSVC runtime
+    // and linking Zig's bundled libc++ would conflict with it.
+    if (target.result.abi != .msvc) {
+        lib.linkLibCpp();
+    }
     lib.addIncludePath(b.path("vendor"));
 
     if (target.result.os.tag.isDarwin()) {

pkg/utfcpp/build.zig

@@ -12,7 +12,11 @@ pub fn build(b: *std.Build) !void {
         }),
         .linkage = .static,
     });
-    lib.linkLibCpp();
+    // On MSVC, the C++ standard library is provided by the MSVC runtime
+    // and linking Zig's bundled libc++ would conflict with it.
+    if (target.result.abi != .msvc) {
+        lib.linkLibCpp();
+    }
 
     if (target.result.os.tag.isDarwin()) {
         const apple_sdk = @import("apple_sdk");