WIP: Fix option namespace pollution

[shadowspawn]

Work in progress. Added a feature flag to get new behaviour, where option values are not stored on the Command object but in a separate field and accessed using new accessor methods. The new behaviour is opt-in, unchanged clients get unchanged old behaviour for first release.

Builds on work in #515 and #673

Feedback welcome (from anyone). Still a work in progress, I am still coding to try ideas so anything could change. 🛠

I will move to branch on tj/commander if other maintainers want easier access to code.

Still under consideration:

1. should new accessor methods be get+has (like Map) or getOption+hasOption for clarity? get/has for simplicity, like original properties
2. better name for mapStyleOptions? storeOptionsAsProperties
3. setFeatureFlag or setFeatureFlags? setFeatureFlags

Still to prototype:

• special case get/has/setFeatureFlag options so they do not affect existing clients who do not opt-in
• pass feature flag to commands so new behaviour is global
• Typescript declarations for new routines
• README
• include existing .opts() in README description of options too
• tests for new behaviour (and tests expecting old behaviour do not change)

Code fragment:

program
   .setFeatureFlags("storeOptionsAsProperties:false")
   .option("-o, once", "description");

if (program.has("once") console.log("Once upon a time");

Other changes:

• took version out of new-behaviour option values, because it is not really an option value (?)
• added warning for an old-behaviour option when can detect there is a conflict
• used null object as prototype for new-behaviour options (so close to a pure map)

[shadowspawn]

A side note: if we did not already have the somewhat undocumented .opts() routine I would have suggested having a property called .opts! Only two extra characters and arguable better separation to have a routine than a property, but I am used to process.argv et al.

[shadowspawn]

My current thinking is:

• a new map property is the canonical source of truth for option values. Name undecided.
• do not add .get and .has to command, instead use map property
• configuration to enable/disable legacy options-as-properties
• keep existing .opts() to reduce breaking changes?

Considering:

• configuration to control whether .action handler is passed Command, options object, or map

[shadowspawn]

I have some new ideas and going to start again, from latest code.