ropensci/targets: Improved crew integration

targets 1.2.0 crew integration Do not assume S3 classes when validating crew controllers. Suggest a crew controller in the _targets.R file from use_targets(). Make tar_crew() compatible with crew >= 0.3.0. Rename argument terminate to terminate_controller in tar_make(). Add argument use_crew in tar_make() and add an option in tar_config_set() to make it configurable. Write progress data and metadata in target_prepare(). Other improvements Allow users to set the default label and level_separation arguments through tar_config_set() (#1085, @Moohan)