fix: Improve compatibility of AsyncIterable helpers for polyfills (Babel/Hermes related)

[kitten]

Summary

This replaces Symbol.asyncIterator with a polyfill implementation, similar to Symbol.observable. It also refactor toAsyncIterable to return an object implementing both the AsyncIterator and AsyncIterable shape.

This change should improve compatibility with async iterator polyfills, specifically Babel transpilations, since it won't require the Symbol.asyncIterator polyfill to be included manually and can deal with how async iterable code is transpiled by Babel, specifically for the Hermes engine in React Native.

Set of changes

• Update toAsyncIterable to also implement AsyncIterator shape
• Add asyncIteratorSymbol polyfill to helpers
• Move observableSymbol to helpers