Add count method

[odcey]

Hi ! Looks like there was a count method on the previous repository https://github.com/textileio/js-threaddb/blob/main/src/local/collection.ts#L202

And still available in the documentation. https://textileio.github.io/js-threaddb/classes/collection.html#count

But not in the latest repository. Am i missing something ?

[odcey]

Didn't realised the new doc was here: https://docs.textile.io/threads/#query