Add function to exit cleanly

New feature

In order for Nextflow to play nicely in managed environments, we need to avoid using System.exit calls, and the recently depreciated exit() Nextflow function.

For problems with pipeline execution, we can use the error() Nextflow function, however there is no equivalent for exiting cleanly if we just want to stop without any problems.

Usage scenario

Example current usage of exit 0 in nf-core pipelines, after printing usage information to the terminal:

if (params.help) {
   log.info paramsHelp("nextflow run my_pipeline --input input_file.csv")
   exit 0
}

Suggest implementation

Equivalent of error() but with exit code 0 and no terminal output. Any supplied strings should not be coloured red.

No implementation file or test is named; start by locating the existing error() and deprecated exit() function implementations in Nextflow. Add a clean-exit function that stops with code 0, produces no terminal output, and does not color supplied strings red, then add coverage for the usage scenario described.