Use WP Autoloader class

[obenland]

This is one of those changes I'm on the fence about as it doesn't strictly improve things, it just makes them "different".
The Autoloader class proposed here is based on Dion's work for the WP.org Plugin Directory and used in its proposed form in the /lib/atomic-v2 library on Dotcom.

I was positively surprised that it seems to just be a drop-in replacement for the current Autoloader, which I didn't quite expect :)
It currently lacks the debug log for when a class is not readable or couldn't be loaded. Was that triggered at any point before?

Proposed changes:

• Replaces inline Autoloader with a separate Autoloader class that can be re-used.
• Moves the integrations loader to be included with the other includes.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Make sure everything still works?

[pfefferle]

I like it and it looks way cleaner

[pfefferle]

Maybe escaping the global functions!?!

[obenland]

Maybe escaping the global functions!?!

What do you mean with that?

[pfefferle]

Why have I used "escape" and I have no idea how it is called 🫣... I mean \spl_autoload_register instead of spl_autoload_register.