Add Atomics ns + .isLockFree

[MaxGraey]

See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Atomics/isLockFree

Also: WebAssembly/design#300 (comment)

[dcodeIO]

I think this should live in a separate Atomics namespace implementing what is available in JS only, somewhat similar to what we do with Math.

[MaxGraey]

Will do

[dcodeIO]

Thanks! :)