Fixes #6727

The major change in this commit is to consolidate all the C imports in
a single decl in main.zig. This is required for Zig 0.14. Without it,
the problem in #6727 will happen. I was never able to minimize why this
happens in order to open a Zig bug.

Beyond this, I fixed the build.zig and build.zig.zon to work with Zig
0.14 so that we can test building `pkg/macos` in isolation. There are no
downstream impacting changes in the build.zig files.

workaround Zig issue: https://github.com/ziglang/zig/issues/16290

* update zig

* pkg/fontconfig: clean up @as

* pkg/freetype,harfbuzz: clean up @as

* pkg/imgui: clean up @as

* pkg/macos: clean up @as

* pkg/pixman,utf8proc: clean up @as

* clean up @as

* lots more @as cleanup

* undo flatpak changes

* clean up @as