Deprecate defaulting to scalar in broadcast (take 2) (#26435)
* fixup Pkg3 broadcasting over iterables
* Default to iterating over broadcast arguments
Broadcast now calls a special helper function, `broadcastable`, on each argument. It ensures that the arguments support indexing and have a shape, or otherwise have defined a custom `BroadcastStyle`.
* Add a deprecation and comment out future behavior. Add NEWS and docs.
