[PR] Add WebAssembly libdefs
[PR] Add WebAssembly libdefs

Summary:
Although WebAssembly is still in a working draft stage, its API is *stable* enough. Typescript types can be found [here](https://github.com/01alchemist/webassembly-types/blob/master/webassembly.d.ts).

Also, I am willing to add some tests.
Pull Request resolved: https://github.com/facebook/flow/pull/5837

Reviewed By: gabelevi

Differential Revision: D8889651

Pulled By: mrkev

fbshipit-source-id: 86514ab9a4ffddfd7c19308190b76e785933691e

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