Promote API code to Zed\Engines\API engine

The functions in api_helpers.php are now the Response class.
The functions in cerbere.php are now the Cerbere trait, included in Response.

To play nice with dependency injection, it implements the WithDatabase trait.
But requests are still read from the server superglobals, through code
published from Zed into Keruald OmniTools previously.

Refactor XML and string format code.

The BeautyXML code is superseded by a call to DomDocument.
As such, this change adds the dom extension as dependency.

Test Plan:
Unit tests added for new code in Zed\Engines\API\Format classes.