More tests and CI updates

• Updated dependencies.
• Improved closest selector mechanism.
• Tests hardened.
• CI improvements.

Small outstanding features implemented

Thanks to Álvaro Guimarães for most of the work in this release.

• HTMLCollection is now Countable.
• HTMLCollection implements ArrayAccess methods.
• Element::closest method.
• HTMLDocument can be constructed from existing native DOMDocument.
• Upgraded dependencies.
• Test coverage increased greatly.

Implement HTMLDocument properties

Implement missing properties such as:

• $document->title
• $document->images
• $document->anchors

PHPDocs on all classes

All DOM interfaces that are currently within the repo are marked up with their PHPDoc blocks.

Certain methods have been moved to their correct locations and tests adjusted accordingly.

Basic DOM implementation

The basics of the modern DOM implementation we have been using within browsers.

There are a few known gaps in functionality, and tests need to be hardened before a production release.