Add default value display to CLI help text

[Copilot]

CLI help text now displays default values for flags, eliminating the need to consult source code or documentation for default behavior.

Changes

• Help text generation (packages/jsdoc-cli/lib/help.js): Appends default values to flag descriptions with priority for defaultDescription over default property. Objects are JSON-serialized; other values use String() conversion. Null check prevents stringifying non-object defaults.

• Test coverage (packages/jsdoc-cli/test/specs/lib/engine.js): Added tests verifying default values display correctly, custom descriptions take precedence, and flags without defaults remain unchanged.

Example output

--access <value>      Document only symbols with the specified access level.
                      Accepts these values: all, package, private, protected,
                      public, undefined Default: All except `private`

--destination <value> The output directory. Default: ./out

--encoding <value>    The encoding to assume when reading source files. 
                      Default: utf8

--help                Print help information and exit.

Handles edge cases: 0, false, "", null, and complex objects via JSON serialization.

Original prompt

Pull request: https://github.com/Goo78/jsdoc/pull/130/files

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