Support argument processing without action handler

[shadowspawn]

Pull Request

Problem

The new processing of arguments is useful in a single command program without an action handler, but
argument defaults and custom processing and choices were only being applied if there was an action handler.

Prompted by .choices() not working without action handler.

Solution

Refactor and process command-arguments whether or not action handler.

When adding the functionality in #1508 I had decided that I preferred not to modify this.args with processed results.
Save processed arguments as this.processedArgs so accessible for simple program without action handler, and from the new hook support too.

[shadowspawn]

Ready for review except I don't want to land anything until after #1522, so marking as draft.

(Started working on this on the wrong branch! 🤦 )

[shadowspawn]

Redo Pull Request after code refactor. New PR is #1529