fix: forcibly enable FinalizationRegistry and iterator helpers

Probably due to a bug in the build system, we could not use `JS::Prefs::setAtStartup_weakrefs(true);` or `JS::Prefs::setAtStartup_experimental_iterator_helpers(true);`:
```
undefined symbol: _ZN2JS5Prefs9weakrefs_E
```

Further more, the file `js/PrefsGenerated.h` does not get copied into the installation location of SpiderMonkey, but stays in `/js/src/_build/js/public/PrefsGenerated.h`.

setup.sh

@@ -83,6 +83,8 @@ sed -i'' -e 's/bool Unbox/JS_PUBLIC_API bool Unbox/g' ./js/public/Class.h
 sed -i'' -e 's/bool js::Unbox/JS_PUBLIC_API bool js::Unbox/g' ./js/src/vm/JSObject.cpp  # same here
 sed -i'' -e 's/shared_lib = self._pretty_path(libdef.output_path, backend_file)/shared_lib = libdef.lib_name/' ./python/mozbuild/mozbuild/backend/recursivemake.py
 sed -i'' -e 's/if version < Version(mac_sdk_min_version())/if False/' ./build/moz.configure/toolchain.configure # do not verify the macOS SDK version as the reqiuired version is not available on Github Actions runner
+sed -i'' -e 's/return JS::GetWeakRefsEnabled() == JS::WeakRefSpecifier::Disabled/return false/' ./js/src/vm/GlobalObject.cpp # forcibly enable FinalizationRegistry
+sed -i'' -e 's/return !IsIteratorHelpersEnabled()/return false/' ./js/src/vm/GlobalObject.cpp # forcibly enable iterator helpers
 cd js/src
 mkdir -p _build
 cd _build