Add Entity for adding Physical Constants like Avogadro

[rocky]

https://reference.wolfram.com/language/ref/Entity.html

[TiagoCavalcante]

@rocky maybe create a _Entity object with the wanted property being a list and Entity access that object?

e.g.:

_Entity := <|
    "Country" -> {"United States", "Argentina", "Brazil"},
    "State" -> {"New York"}
|>