Make command-argument array public (in readonly mode)

[aweebit]

Problem

The array of registered command-arguments (currently ._args) is not made publicly available unlike the arrays of registered options (.options) and subcommands (.commands). That is not only an obvious inconsistency, but also hinders third-party library development.

Solution

Rename ._args to .registeredArguments (because .arguments is already used for a method) and add the field to the type declaration for Command in readonly mode.

Other names considered:

• .commandArguments: confusing since the word "command" is not included in the names of .argument(), .arguments() and .addArgument()
• .addedArguments: makes it seem like only arguments added vis .addArguments() are included

The use of the word "registered" is also consistent with the name of ._registerOption(), an internal routine suggested in #1923 and #1931.

ChangeLog

Added

• .registeredArguments (array with Argument instances for all registered command-arguments)

[shadowspawn]

This fixes #1823

registeredArguments is potentially better than declaredArguments which was the best I came up with in:
#1823 (comment)

[shadowspawn]

To avoid immediately breaking existing client code which may have directly accessed _args, I suggest this._args = this.registeredArguments in constructor (and deprecating _args to remove later).

[shadowspawn]

Suggested change

	readonly _args: readonly Argument[]; // added here for strikethrough highlighting in editors
	readonly _args: readonly Argument[];

[shadowspawn]

Use same style as other deprecated messages

Suggested change

	/** @deprecated Use `.registeredArguments` instead. */
	/** @deprecated since v11, instead use `.registeredArguments` */

[aweebit]

I actually thought about specifying the version, but decided against it because the property had never been supported officially.

[shadowspawn]

Looks noisy because the name is so much longer! But not many authors will be typing it, and best name so far (and get away from the existing args/arguments/arguments). Should be accessible for symmetry with commands and options for people adding functionality.

Couple of minor changes for the comments adding and deprecating _args to typings. (Which surprised me at first, but happy now.)

(I'll update deprecation documentation later to more closely follow match the style I am using.)

[shadowspawn]

Closing in favour of #2010, where will list @aweebit as co-author.