build: add GhosttyVt module map to xcframework and Swift example

The xcframework now generates its own headers directory with a
GhosttyVt module map instead of reusing include/ directly, which
contains the GhosttyKit module map for the macOS app. The generated
directory copies the ghostty headers and adds a module.modulemap
that exposes ghostty/vt.h as the umbrella header.

A new swift-vt-xcframework example demonstrates consuming the
xcframework from a Swift Package. It creates a terminal, writes
VT sequences, and formats the output as plain text, verifying
the full round-trip works with swift build and swift run.