fix: accessor descriptors from Proxy getOwnPropertyDescriptor trap

[NathanWalker]

js_obj_to_desc() returns defineProperty-style flags (JS_PROP_HAS_GET / JS_PROP_HAS_SET, never JS_PROP_GETSET), but js_proxy_get_own_property() published the parsed trap result unconverted. Descriptor consumers test JS_PROP_GETSET, so any accessor descriptor returned by a getOwnPropertyDescriptor trap degraded to a data descriptor { value: undefined, writable: false }.

Convert the flags to own-property form before publishing, per the CompletePropertyDescriptor(resultDesc) step of Proxy [[GetOwnProperty]]:

    const t = {};
    Object.defineProperty(t, 'x', { configurable: true, get() { return 42; } });
    const p = new Proxy(t, { getOwnPropertyDescriptor: Reflect.getOwnPropertyDescriptor });
    Object.getOwnPropertyDescriptor(p, 'x');
    // before: { value: undefined, writable: false, ... }
    // after:  { get: [Function], set: undefined, ... }

Embedders exposing host objects through descriptor-trap proxies hit this in practice (eg, NativeScript's iOS runtime did).