KDL 2.0 compliance

[zkat]

KDL 2.0.0-draft.3 is now generally available (kdl-org/kdl#286), and with it, we've decided to encourage existing implementations to start working towards 2.0 support and send feedback and suggestions for anything you might run into, so I'm making this tracking issue. :)

[zkat]

KDL 2.0.0 has been released (https://github.com/kdl-org/kdl/releases/tag/2.0.0)

[shieldo]

It's been a wee while since I've touched PHP (I'm mainly a Rust human these days) but, on it!