Replace deprecated Hooks::register / Hooks::isRegistered / Hooks::getHandlers

[umherirrender]

Issue

The functions Hooks::register / Hooks::isRegistered / Hooks::getHandlers are soft-deprecated and going to emit warnings, see
https://phabricator.wikimedia.org/T335536

[paladox]

Fixed with #33.

Although getHandlerCallbacks is deprecated and I don't know what to replace it with.

And I couldn't use getHandlers within MW 1.39 from HookContainer as it was causing tests to fail. So had to keep the deprecated Hooks::getHandlers for that release.