libghostty: Remove all libc++ and libc++ ABI dependencies

This updates simdutf to my fork which has a SIMDUTF_NO_LIBCXX option
that removes all libc++ and libc++ ABI dependencies. 

From there, the hand-written simd code we have has been updated to also
no longer use any libc++ features. Part of this required removing utfcpp
since it depended on libc++ (`<iterator>`).

libghostty-vt now only depends on libc.

example/cpp-vt-stream/build.zig

@@ -14,6 +14,7 @@ pub fn build(b: *std.Build) void {
         .root = b.path("src"),
         .files = &.{"main.cpp"},
     });
+    exe_mod.link_libcpp = true;
 
     // You'll want to use a lazy dependency here so that ghostty is only
     // downloaded if you actually need it.

example/swift-vt-xcframework/Package.swift

@@ -8,10 +8,7 @@ let package = Package(
         .executableTarget(
             name: "swift-vt-xcframework",
             dependencies: ["GhosttyVt"],
-            path: "Sources",
-            linkerSettings: [
-                .linkedLibrary("c++"),
-            ]
+            path: "Sources"
         ),
         .binaryTarget(
             name: "GhosttyVt",