Unexpose internal data property of AnimationLibrary

[YuriSizov]

Supersedes #82713, see the discussion there.

Underscored properties should not be exposed in the first place, so this was a mistake in either the name or property flags. In either case fixing this breaks compat, and it's safer to assume this was not supposed to be exposed.

[AThousandShips]

I believe all this changes is:

• No longer documented
• Inaccessible through extensions and C# (I think)

I'm not sure if the methods are now exposed in C# (normally a property blocks them) or extension

But it's supposed to be internal anyway so I'd say worth it

Edit: correction, already hidden in extensions as it checks leading _, and doesn't consider this flag, nor does C# I think

[YuriSizov]

Thanks for reviews!