This PR adds a new cop called `Rails/PrivateTransactionOption`.

This cop checks whether `ActiveRecord::Base.transaction(joinable: _)`
is used. The `joinable` option is a private API and is not intended
to be called from outside Active Record core.

rails/rails#39912 (comment)
rails/rails#46182 (comment)

Passing `joinable: false` may cause unexpected behavior such as the
`after_commit` callback not firing at the appropriate time.

Co-authored-by: Phil Pirozhkov <[email protected]>