Warn on pending block

Not sure this is fully finished yet. I wanted to start the review process sooner rather than later.

Things which I'm not sure about:

[ ] To on my part: Add `deprecated` relish doc for this
[ ] Does this need a Changelog entry?
[ ] Should I make a similar change in a new PR for 2.99?

When going in to do testing I was a little confused on where to put the spec. There are a few places that `pending` is used in specs:

* `spec/rspec/core/example_group_spec.rb`
* `spec/rspec/core/example_spec.rb`
* `spec/rspec/core/pending_example_spec.rb`
* `spec/rspec/core/pending_spec.rb`

I first looked in `spec/rspec/core/pending_spec.rb` but weren't really any specs in. Since this change is specific to `pending` when used in an example, I then checked `spec/rspec/core/example_spec.rb`. Some of the basic behavior was spec'd in there, but a bit of the subtle behaviors were not. I settled on `spec/rspec/core/pending_example_spec.rb` as this seemed to have specs around the majority of the example specific behavior.