Add api exposing a hook-based way to load a module

[edkimmel]

Summary

Adds loadable.hook and lazy.hook as additional ways to load a dynamic module via @Loadable

A common use case is to use some large third party library as part of a hook in first party code. By exposing a hook-based API in @Loadable, it becomes trivial to dynamically load that library module.

const useThirdPartyAuthSdk = loadable.hook(() => 'some-large-sdk')
...

const useUserSignIn = () => {
  // 'some-large-sdk' is not loaded until this hook is ran
  const thirdPartyAuthSdk = useThirdPartyAuthSdk()
  const signin = React.useCallback(creds => {
    if (!thirdPartyAuthSdk) throw new Error('Auth SDK used before ready')
    thirdPartyAuthSdk.initiateSignin(creds)
  }, [thirdPartyAuthSdk])
  return {
    ready: !!thirdPartyAuthSdk,
    signin
  }
}

Test plan

loadableHook.test.js has been created mirroring loadable.test.js where applicable.
An example for the hook api was created in the repository.

[theKashey]

Amazing job and quite interesting implementation. Big kudos for comprehensive tests.

However, let me be clear - I am going to add this PR to the list of all other things one way or another assigned to me and I cannot provide you any ETA for actual resolution. My presence in GitHub lands is quite limited nowadays.