Test toolchain against test262

Work in progress integration of the [test262](https://github.com/tc39/test262) test suite into Ezno. Currently this PR just checks whether sources pass (or not according to `negative` flag. In the future it would be good to check - `.to_string()` roundtrip (as we do in fuzzing) - That accepted files run in the type checker - That negative runtime tests get picked up by the type checker (experiment)