Currently `ActiveModel::Model` is defined as the minimum API to talk
with Action Pack and Action View.
Its name suggests it can be included to create Active Record type
models, but for creating models it's probably too minimal. For example
it's very common to include ActiveModel::Attributes as well.

By moving `ActiveModel::Model`'s implementation to a new
`ActiveModel::API` we keep a definition of the minimum API to talk with
Action Pack and Action View.

For `ActiveModel::Model` we only need to include `ActiveModel::API`.
This will allow adding more funcationality to `ActiveModel::Model` while
keeping backwards compatibility.

Co-authored-by: Nathaniel Watts <1141717+thewatts@users.noreply.github.com>