Add newlines after too many deprecations warnings

This is for #1147

I liked your heredoc pipe trick from a recent commit, so I re-used it here.

I was also considering just adding a newline between deprecations of different types (regardless of whether or not there is a `too_many_warnings_message`. Currently deprecation messages are stored in a hash of `deprecation type => array of messages` and we just flatten all the values to print them, but we could easily iterate over the keys and add newlines after each deprecation type.

What do you think?