Zig 0.15 removed the ability to compress from the stdlib, which makes
porting our framegen tool to Zig 0.15+ more work than it's worth. We
already depend on and have the ability to build zlib, and Zig is a full
blown C compiler, so let's just use C.

The framegen C program doesn't free any memory, because it is meant to
exit quickly. It otherwise behaves pretty much the same as the old Zig
codebase.

The build scripts were modified to build the C program and run it, but
also to include the framedata in the generated source tarball so that
downstream packagers don't have to do this (although they'll have all
the deps anyways).

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

This introduces the concept of a "dist resource" (specifically a
`GhosttyDist.Resource` type). This is a resource that may be present in
dist tarballs but not in the source tree. If the resource is present and
we're not in a Git checkout, then we use it directly instead of
generating it.

This is used for the first time in this commit for the gresource c/h
files, which depend on a variety of external tools (blueprint-compiler,
glib-compile-resources, etc.) that we do not want to require downstream
users/packagers to have and we also do not want to worry about them
having the right versions.

This also adds a check for `distcheck` to ensure our distribution
contains all the expected files.

This moves the source tarball creation process into the Zig build system
and follows the autotools-standard naming conventions of `dist` and
`distcheck`.

The `dist` target creates a source tarball in the `PREFIX/dist`
directory. The tarball is named `ghostty-VERSION.tar.gz` as expected by
standard source tarball conventions.

The `distcheck` target does the same as `dist`, but also takes the
resulting tarball, extracts it, and runs tests on the extracted source
to verify the source tarball works as expected.

This commit also updates CI:

  1. Tagged releases now use the new `zig build distcheck` command.
  2. Tip releases now use the new `zig build dist` command.
  3. A new test build tests that source tarball generation works on
     every commit.