Introduce mw.wikibase.entity Lua library

This is a new and fancier way to handle entities in Lua.
It's very basic at this step and will be extended in other
commits.

Bug: 54324
Change-Id: Icd8d5087cd4feea0e11f918afd16d906d396cc30
Merge "Introduce mw.wikibase.entity Lua library"