Rename all private properties to start with _underscore?

[shadowspawn]

We have some properties (including functions) on Command which are intended to be private but do not start with an underscore.

Advantages of renaming them are:

• make it clearer that they are private
• reduce noise in code completion suggestions in editors
• reduce name collisions when storing option values as properties on Command

We would treat this as a breaking change and a major version bump, since some clients will be using private properties whether by accident or design.

I am interested if many readers are using or overriding private properties. I suspect some clients customise the error display via missingMandatoryOptionValue et al.

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For interest, there is a proposal for true private fields in javascript:

• https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md

[shadowspawn]

I like the idea of cleaning up the autocompletion, but no one has complained. Since it would break anyone currently using a private property, not confident the cost/benefit is worthwhile for now.