[nooga]

Part of #72 (group A: global var bindings and the global object).

Problems

• Global var and function bindings lived only in heap slots, and the global object got copies only after an indirect eval. As a result, Object.getOwnPropertyDescriptor(globalThis, "x") returned undefined for a plain script var x.
• hasOwnProperty answered true for top-level let/const/class.
• var x; inside eval (direct or indirect) reset an existing global x to undefined.
• Eval never ran the CanDeclareGlobalFunction or CanDeclareGlobalVar checks.

Fix

• Var and function slots are now mirrored onto the global object as data properties. The heap's slot watch keeps them current; this is the same mechanism module namespaces use (pkg/vm/global_bindings.go).
  • Script declarations are non-configurable.
  • Eval declarations and implicit globals (zz = 1) are configurable.
  • Deleting a binding drops its mirror.
• Top-level classes initialize their slot with OpSetGlobalInit, like let, so they are no longer mistaken for implicit globals.
• A new OpDeclareGlobal implements CreateGlobalVarBinding and CreateGlobalFunctionBinding for eval code. In check mode it is emitted for every name before any binding is created, so a non-definable name throws a TypeError and declares nothing.
• The blanket SyncHeapToGlobalObject after indirect eval is removed. It also copied lexical bindings onto the global object.

Module top-level bindings are not mirrored.

Testing

• New smoke test tests/scripts/global_var_bindings_on_global_object.ts. TestScripts is green.
• test262 (language + built-ins, vs main): +18, no regressions. This covers:
  • eval-code/{direct,indirect}/var-env-*-init-global-*
  • non-definable-*
  • global-code/decl-{func,lex,var}

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