Export error codes from Auth

[Feiyang1]

Export error codes as an object map from Auth to make error code check easier. Addresses #4551
To enable use case like the following:

// ErrorCodes is a map of all possible Auth error codes
import { signInAnonymously, ErrorCodes } from 'firebase/auth';

async function signIn() {
    try {
    const credential = signInAnonymously(auth);
  } catch (e) {
    // instead of e.code === 'auth/network-request-failed'
    if (e.code === ErrorCodes.NetworkRequestFailed) {
      // do stuff
    }
  }
}

Object maps are tree shakable, so they incur no size cost if they are not imported.

We may want to do the same in other SDKs as well.