Upstream is now mostly pure Zig and the build.zig.zon.* files are
generated directly by zon2nix. The JSON file is no longer used as an
intermediate file but is retained for downstream packager usage.

Closes #6702

This removes our mach-glfw dependency and replaces it with an in-tree
pkg/glfw that includes both the source for compiling glfw as well as the
Zig bindings. This matches the pattern from our other packages.

This is based on the upstream mach-glfw work and therefore includes the
original license and copyright information.

The reasoning is stated in the issue but to summarize for the commit:

  - mach-glfw is no longer maintained, so we have to take ownership
  - mach-glfw depended on some large blobs of header files to enable
    cross-compilation but this isn't something we actually care about,
    so we can (and do) drop the blobs
  - mach-glfw blobs were hosted on mach hosts. given mach-glfw is
    unmaintained, we can't rely on this hosting
  - mach-glfw relied on a "glfw" package which was owned by another
    person to be Zig 0.14 compatible, but we no longer need to rely on
    this
  - mach-glfw builds were outdated based on latest Zig practices

This fixes a regression in 1.1.1/1.1.2 where our PACKAGING docs mention
using `fetch-zig-cache.sh` but it was removed. This commit adds it back,
generating its contents from the build.zig.zon file (via zon2nix which
we use for our Nix packaging).

For packagers, there are no dependency changes: you still need Zig and
POSIX sh. For release time, Ghostty has a new dependency on `jq` but
otherwise the release process is the same. The check-zig-cache.sh script
is updated to generate the new build.zig.zon.txt file.

This brings the internal package more in line with how the nixpkgs
package is built. It also handles recursive dependencies better than the
current system.