Function js::jsapi::JS_IsExtensible

pub unsafe extern "C" fn JS_IsExtensible(
    cx: *mut JSContext, 
    obj: HandleObject, 
    extensible: *mut bool
) -> bool

Determine whether obj is extensible. Extensible objects can have new properties defined on them. Inextensible objects can't, and their [[Prototype]] slot is fixed as well.

Implements: ES6 [[IsExtensible]] internal method.