extra/dart: add wrapper exposing SDK
This commit adds a wrapper script around the Dart binaries in the (highly shortened) style of the Flutter SDK.
The compiled Dart binary listens to the environment variable
FLUTTER_ROOT
which is computed in the shared.sh script of the Flutter SDK. Such a wrapper was as of now missing in the Arch Linux extra/dart package causing Flutter installations to fail using the distribution-provided Dart installation.
Signed-off-by: The one with the braid info@braid.business