Reverts 4861015 ("feat(macos): foundation of the macOS app ported from
Swift to Rust"). Decision with the user: keep Ghostty's original Swift
macOS app unchanged and link it against the Rust core instead of the
Zig core. The Rust libghostty exports the identical include/ghostty.h
ABI (ABI-checked), so the Swift app does not need to be rewritten; a
second, Rust-based macOS app would duplicate it without being runnable
or testable here.