ArgParser constructor
Creates a new ArgParser.
If allowTrailingOptions is true (the default), the parser will parse
flags and options that appear after positional arguments. If it's false,
the parser stops parsing as soon as it finds an argument that is neither
an option nor a command.
Implementation
factory ArgParser({bool allowTrailingOptions = true, int? usageLineLength}) =>
ArgParser._(<String, Option>{}, <String, ArgParser>{}, <String, String>{},
allowTrailingOptions: allowTrailingOptions,
usageLineLength: usageLineLength);