[6.x] Support Laravel Debugbar 4

[duncanmcclean]

This pull request adds return types and a method_exists() check necessary to support Laravel Debugbar 4.

Fixes #13780

[jasonvarga]

The Antlers tab is broken. It's using jQuery, which has been removed in debugbar 4.
That's acceptable for now. If you need it, downgrade to debugbar 3.
We can fix it separately. This at least lets people use debugbar 4 for everything else in the mean time.