Add support for callable class instances #494

TypeError: [object name] is not a function

import pythonmonkey as pm
def test_func():
 print("test_func")
class test_class:
 def __call__(self):
 print("test_class")
globalThis = pm.eval("globalThis;")
globalThis.test_func = test_func
globalThis.test_class_instance = test_class()
pm.eval("globalThis.test_func()") # This works
pm.eval("globalThis.test_class_instance()") # This does not work

Traceback (most recent call last):
 File "[PATH TO FILE].py", line 17, in <module>
 pm.eval("globalThis.test_class_instance()") # This does not work
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pythonmonkey.SpiderMonkeyError: Error in file evaluate, on line 1, column 12:
TypeError: globalThis.test_class_instance is not a function
Stack Trace:
 @evaluate:1:12