Sync and ASync versions of function interfaces #38

As pointed out by @huningxin (at https://chromium-review.googlesource.com/c/chromium/src/+/3684745), we may need to have two versions of some functions like createContext. The WebNN spec already has it --- we should be aligned with it.

We should discuss whether and how to achieve it (e.g. who should be the default, the "sync" or the "async" version?)