Feature Request - Improve Async JavaScript Binding Support

- [ ] Generate get/set methods for properties that return simple type bool/string etc
- [ ] Generate sub bound object for properties that return complex types allowing for their methods to be called.
- [x] Add support for methods that return `Task` (See #2758)

New issues will be created to track the issues independently when it's time to implement them.

This should bring the Async version in line with the sync version feature wise.

If/When. NET core is support we will have to conditionally exclude the sync version as it relies on WCF which Microsoft have recently announced will not be included.