Support async: false

Some very old legacy code may be dependent on having synchronous AJAX. While we don't recommend using it for any current for future development, it is now supported in can-ajax. To use synchronous AJAX, the async property must be explicitly set to false.

For synchronous calls, Promises still resolve in microtasks as with asynchronous AJAX, but success and error callbacks passed as properties to ajax() are resolved in thread. This means that success and error callback properties are now officially part of the public API as well. They were previously supported but undocumented.