ENH: allow EA to register types for is_scalar
https://github.com/pandas-dev/pandas/pull/27461#discussion_r305168936

> i think we need. way for EA to hook into this for an EA scalar
> eg an IPaddress from cyberpandas could register a scalar i think

Before we move on this, I think we need to clarify in which situations we care about `lib.is_scalar(x)` vs the simpler `np.ndim(x) == 0`