A user complained about getting a misleading error after a typo in the
mnemonic. The word was `hear` and it passed the check even when it is
not in the list of valid words.
The reason is that we where checking if the word is in the variable
`englishWordList` (which includes all the words) instead of checking if the
variable is in the `defaultWordList` (which is basically `englishWoldList`
split by spaces). That means that `hear` passed the check because `heart`
appears in the list.
Related issue [4733](https://github.com/lightningnetwork/lnd/issues/4733)
In this commit, we publicly export the `word` field as it makes it
easier to programmatically interact with the package when attempting to
re-derive proper `cipherseed` instances. We also add a new `Index` field
as well to provide additional context for programmatic manipulating of
seeds.
