libghostty: add cpp-vt-stream example and fix C++ header compatibility

Add a cpp-vt-stream example that verifies libghostty headers compile
cleanly in C++ mode. The example is a simplified C++ port of
c-vt-stream.

The headers used the C idiom `typedef struct Foo* Foo` for opaque
handles, which is invalid in C++ because struct tags and typedefs
share the same namespace. Fix all 12 opaque handle typedefs across the
headers to use a distinct struct tag with an Impl suffix, e.g.
`typedef struct GhosttyTerminalImpl* GhosttyTerminal`. This is a
source-compatible change for existing C consumers since the struct
tags were never referenced directly.

example/cpp-vt-stream/src/main.cpp

@@ -0,0 +1,49 @@
+#include <cassert>
+#include <cstdio>
+#include <cstring>
+#include <ghostty/vt.h>
+
+int main() {
+  // Create a terminal
+  GhosttyTerminal terminal;
+  GhosttyTerminalOptions opts = {
+    .cols = 80,
+    .rows = 24,
+    .max_scrollback = 0,
+  };
+  GhosttyResult result = ghostty_terminal_new(nullptr, &terminal, opts);
+  assert(result == GHOSTTY_SUCCESS);
+
+  // Feed VT data into the terminal
+  const char *text = "Hello from C++!\r\n";
+  ghostty_terminal_vt_write(terminal, reinterpret_cast<const uint8_t *>(text), std::strlen(text));
+
+  text = "\x1b[1;32mGreen Text\x1b[0m\r\n";
+  ghostty_terminal_vt_write(terminal, reinterpret_cast<const uint8_t *>(text), std::strlen(text));
+
+  text = "\x1b[1;1HTop-left corner\r\n";
+  ghostty_terminal_vt_write(terminal, reinterpret_cast<const uint8_t *>(text), std::strlen(text));
+
+  // Get the final terminal state as a plain string
+  GhosttyFormatterTerminalOptions fmt_opts =
+      GHOSTTY_INIT_SIZED(GhosttyFormatterTerminalOptions);
+  fmt_opts.emit = GHOSTTY_FORMATTER_FORMAT_PLAIN;
+  fmt_opts.trim = true;
+
+  GhosttyFormatter formatter;
+  result = ghostty_formatter_terminal_new(nullptr, &formatter, terminal, fmt_opts);
+  assert(result == GHOSTTY_SUCCESS);
+
+  uint8_t *buf = nullptr;
+  size_t len = 0;
+  result = ghostty_formatter_format_alloc(formatter, nullptr, &buf, &len);
+  assert(result == GHOSTTY_SUCCESS);
+
+  std::fwrite(buf, 1, len, stdout);
+  std::printf("\n");
+
+  ghostty_free(nullptr, buf, len);
+  ghostty_formatter_free(formatter);
+  ghostty_terminal_free(terminal);
+  return 0;
+}