pkg: replace @cImport with addTranslateC in pkg/

@cImport is going to disappear in Zig 0.17. Its deprecated in Zig 0.16.
Let's remove it now.

Replace @cImport with addTranslateC across pkg/ packages. Each
package now has a c_import.h header that is translated at build
time via addTranslateC and exposed as a "cimport" module import.

Converted packages:
- dcimgui
- fontconfig
- freetype
- glslang
- harfbuzz
- macos
- oniguruma
- opengl
- sentry
- spirv-cross
- wuffs

Omitted:
- gtk4-layer-shell - This has a bit more complexity with how it
  interacts with GTK headers, so I need to consider this a bit more.
- src/ - It'll be cleaner to do this separately.

pkg/macos/c_empty.zig

@@ -0,0 +1,2 @@
+// Empty stub for non-Darwin targets where the macOS C headers
+// are not available.