Added install script for Mac Silicon which compiles dart

[c-joly]

• This script is based on the install_osx script. It compiles and install dart / dartpy following the steps provided on the documentation.

• An ugly patch has been added to include <std.io> in a header (maybe it can be avoided if we force cmake to use g++ instead of clang which is used by default?)

• Some environment variables have been changed/added (PYTHONPATH, DYLD_LIBRARY_PATH)

• I did not add any additional installation in the script (like cmake, xquartz or ffmpeg). These commands are all installable with homebrew but they might have been already installed by the user by other means (e.g. cmake and xquartz have they own PKG/DMG installers). To avoid any side effect to the user, I did not add any of these install in the script, but documentation should mention these programs as prerequisite.

[costashatz]

This is fixed! See #201!