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/README.md

@@ -0,0 +1,19 @@
+# Example: VT Stream Processing in C++
+
+This contains a simple example of how to use `ghostty_terminal_vt_write`
+to parse and process VT sequences in C++. This is a simplified C++ port
+of the `c-vt-stream` example that verifies libghostty compiles in C++
+mode.
+
+> [!IMPORTANT]
+>
+> **`libghostty` is a C library.** This example is only here so our CI
+> verifies that the library can be built in used from C++ files.
+
+## Usage
+
+Run the program:
+
+```shell-session
+zig build run
+```