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).