Support argument processing without action handler

[shadowspawn]

Pull Request

Problem

The new processing of arguments is also 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 realising .choices() would not work 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.

This PR includes some minor other improvements:

• added .arg to Command constructor
• initialise .arg to empty array (matches existing TypeScript declaration)
• add JSDoc types for some key Command properties

Changelog

• Changed: Command property .arg initialised to empty array (was previously undefined)

[abetomo]

👍

[shadowspawn]

Thanks @abetomo