add `formatversion` to `WbGetEntities`

[k-yle]

formatversion is undocumented^[1] but it does exist^[2]. Also exported this interface, for anyone that's using this library purely for its type-defintions

[maxlath]

Do you have an example of an API call url that returns that attribute?

[k-yle]

this is attribute is in the request, not the response. It's briefly mentioned in the release notes for MediaWiki 1.40.

as an example, you could compare the response from:

• action=wbgetentities & ids=Q0 & formatversion=1
• action=wbgetentities & ids=Q0 & formatversion=2