Preserve parsed values for prototype-named destinations

[xiehuanyi]

A destination named __proto__ is currently consumed by JavaScript's inherited prototype setter: parsed strings disappear and object defaults change the namespace prototype. For example, parser.add_argument('--value', { dest: '__proto__' }) followed by parse_args(['--value', 'parsed']) produces no own destination attribute.

Define that name as an ordinary own attribute in the shared setter. Use the same setter when initializing a Namespace while preserving enumerable symbols, null sources, and boxed primitive source behavior.

The three new destination/initializer regressions fail before the change. They cover explicit values and object defaults, append/count actions on a supplied namespace, and enumerable symbol copying. Compatibility checks also cover Namespace(null) and Namespace('x').

Validation on Node 22: npm test passes lint and all 1906 tests; git diff --check passes.

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[puzrin]

Garbage