Cleanly expose `superchain` image

[sam-goodwin]

It's currently ugly if you want to use the superchain docker image:

new delivlib.LinuxPlatform(cbuild.LinuxBuildImage.fromAsset(this, 'superchain', {
  directory: path.join(__dirname, '..', 'node_modules', 'aws-delivlib', 'superchain')
}));

Should probably be something like:

new delivlib.SuperChain(this, 'superchain');