Interpreter can't translate `thread` construct to JS yet

[rossberg]

Which means we cannot merge into the main repo yet.

[conrad-watt]

There's an approach for doing this in https://github.com/WebAssembly/threads/tree/extract-js-poc but it's super ugly, so I should clean it up (or maybe do a clean slate).

[danleh]

V8 also currently doesn't include the tests from this repo due to this missing extraction in the spec interpreter. See https://crbug.com/v8/14376.

[rossberg]

@conrad-watt, this proposal still needs some work to move it over the finishing line. ;)