…#20348)

## Summary

Closes #20133

- Lower decorated `accessor` properties to private backing field + getter/setter pair when `experimentalDecorators` is enabled, matching TypeScript's `classFields.ts` `transformAutoAccessor`
- Only accessors with decorators are lowered — undecorated accessors are left as-is
- Support computed accessor keys (e.g. `@dec accessor [expr]`) via temp var to ensure the key expression is evaluated only once
- Accessor lowering runs in `enter_class` (Decorator plugin) so es2022 class-properties can further transform the generated private fields
- Decorators are transferred from the accessor to the generated getter, so the legacy decorator transform emits `_decorate()` calls

### Transform example

```ts
// Input
class C {
  @dec accessor prop: string = "hello";
  @dec accessor [expr]: any;
}
```

```js
// Output (without class-properties)
var _a = expr;
class C {
  #_prop_accessor_storage = "hello";
  get prop() { return this.#_prop_accessor_storage; }
  set prop(value) { this.#_prop_accessor_storage = value; }
  #_a;
  get [_a]() { return this.#_a; }
  set [_a](value) { this.#_a = value; }
}
babelHelpers.decorate([dec], C.prototype, "prop", null);
babelHelpers.decorate([dec], C.prototype, _a, null);
```

## Test plan

- [x] New test `oxc/accessor` — decorated accessors without class-properties plugin (instance, static, initialized, computed key)
- [x] New test `oxc/accessor-with-class-properties` — full pipeline with class-properties plugin (private fields transformed to WeakMap/brand patterns)
- [x] New test `decoratorOnClassAccessorProperty1` — instance, static, and initialized accessor properties with decorators
- [x] Existing `decoratorOnClassProperty13` output updated for accessor lowering
- [x] Undecorated accessors are not lowered (`oxc-13284.js` coverage test passes)
- [x] `cargo clippy`, `just fmt` clean

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