Proof of concept async / await implementation.

[aardvark179]

This stacks on top of #2335 and implements a proof of concept implementation of async based on generators. I have not yet implemented async iterators—these are pretty simple at the JavaScript level, but would really require some native Java support, and an implementation of at least some built in methods.

Claude did write some of this code for me, but within some fairly strong architectural guidelines, and I did tweak a lot of it afterwards.

This PR adds the following commits on top of #2335:

1. Expose $262 in shell for ease of exploratory testing.
2. Refactor away from WithScope for catching exceptions.
3. Update 262 properties.
4. Fix various destructuring issues.
5. Update 262 properties.
6. Add support for async to 262 test runner.
7. Initial async parsing support.
8. Initial 262 properties update.
9. Enable async-functions in 262 tests and update properties.
10. Await proof of concept implementation.
11. Update 262 properties.