support for engine arguments to control threshold structure and parallel regression assumption

[corybrunson]

This PR is coordinated with this parsnip PR and this dials PR. From 'NEWS.md':

The threshold_structure model argument for ordinal_reg() controls what constraints, if any, are imposed on the ordered thresholds. It can be used by the clm and vglm engines.

The parallel_reg model argument for ordinal_reg() provides a unified interface for controlling the parallel regression assumption. It accepts a logical value (applied to all terms), a formula with a logical LHS naming parallel or non-parallel terms, or a list combining both; the engines clm, vglm, and ordinalNet are compatible with different subsets of specifications:

• clm_wrapper() (new) translates the parallel_reg specification to a formula accepted by ordinal::clm(nominal)
• VGAM_vglm_wrapper() and VGAM_vgam_wrapper() accept a parallel_reg argument and translate it for the VGAM parallel argument
• ordinalNet_wrapper() accepts a parallel_reg argument and translates it for the parallelTerms and nonparallelTerms arguments

The PR also includes a tuning dial for the parallel penalty factor, an ordinalNet-specific numeric argument.