Generic field definition method. Similar to flag/flagList/number/etc,
but you must specify the type (and optionally multiple and delim)
fields on each one, or Jack won't know how to define them.
Parse a string of arguments, and return the resulting
{ values, positionals } object.
If an JackOptions#envPrefix is set, then it will read default
values from the environment, and write the resulting values back
to the environment as well.
Environment values always take precedence over any other value, except
an explicit CLI setting.
Only parse the command line arguments passed in.
Does not strip off the node script.js bits, so it must be just the
arguments you wish to have parsed.
Does not read from or write to the environment, or set defaults.
Set the default value (which will still be overridden by env or cli)
as if from a parsed config file. The optional source param, if
provided, will be included in error messages if a value is invalid or
unknown.
Class returned by the jack function and all configuration definition methods. This is what gets chained together.