Implement TC39 Stage 3 decorators and auto-accessors (preliminary)

[lahma]

Summary

Preliminary support for the TC39 Stage 3 decorators proposal. The decorators specification has not yet been formally approved into the ECMAScript standard — this implementation follows the Stage 3 proposal.

Auto-accessors (accessor keyword):

• Public, private, and static auto-accessors with getter/setter pairs
• Backed by per-instance private storage using PrivateName
• Correct override and inheritance semantics

Decorator evaluation:

• Method/getter/setter decorators: receive function value, can return replacement
• Field decorators: receive undefined, can return initializer wrapper
• Auto-accessor decorators: receive {get, set}, can return {get?, set?, init?}
• Class decorators: receive constructor, can return replacement
• addInitializer callback for both instance and static contexts
• Correct application order (last decorator applied first)
• Decorator context object with kind, name, static, private, addInitializer

Test plan

• Full test262: 95,985 passed, 0 failures, no regressions
• test262 decorator staging tests: 34/34 passed (public-auto-accessor, private-auto-accessor, accessor-as-identifier)
• 20 new decorator unit tests covering: method/class/field/accessor decorators, addInitializer, multiple decorators, getter/setter decorators, static decorators, context properties

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