We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

Instead of using @hasDecl, use a performAction-stype API. The C
interface for interfacing with macOS (or any other apprt where Ghostty
is embedded) is unfinished.

This deletes the GLFW apprt from the Ghostty codebase.

The GLFW apprt was the original apprt used by Ghostty (well, before
Ghostty even had the concept of an "apprt" -- it was all just a single
application then). It let me iterate on the core terminal features,
rendering, etc. without bothering about the UI. It was a good way to get
started. But it has long since outlived its usefulness.

We've had a stable GTK apprt for Linux (and Windows via WSL) and a
native macOS app via libghostty for awhile now. The GLFW apprt only
remained within the tree for a few reasons:

  1. Primarily, it provided a faster feedback loop on macOS because
     building the macOS app historically required us to hop out of the
     zig build system and into Xcode, which is slow and cumbersome.

  2. It was a convenient way to narrow whether a bug was in the
     core Ghostty codebase or in the apprt itself. If a bug was in both
     the glfw and macOS app then it was likely in the core.

  3. It provided us a way on macOS to test OpenGL.

All of these reasons are no longer valid. Respectively:

  1. Our Zig build scripts now execute the `xcodebuild` CLI directly and
     can open the resulting app, stream logs, etc. This is the same
     experience we have on Linux. (Xcode has always been a dependency of
     building on macOS in general, so this is not cumbersome.)

  2. We have a healthy group of maintainers, many of which have access
     to both macOS and Linux, so we can quickly narrow down bugs
     regardless of the apprt.

  3. Our OpenGL renderer hasn't been compatible with macOS for some time
     now, so this is no longer a useful feature.

At this point, the GLFW apprt is just a burden. It adds complexity
across the board, and some people try to run Ghostty with it in the real
world and get confused when it doesn't work (it's always been lacking in
features and buggy compared to the other apprts).

So, it's time to say goodbye. Its bittersweet because it is a big part
of Ghostty's history, but we've grown up now and it's time to move on.
Thank you, goodbye.

(NOTE: If you are a user of the GLFW apprt, then please fork the project
prior to this commit or start a new project based on it. We've warned
against using it for a very, very long time now.)