Add wrapper for custom Transforms

[WebMechanic]

Currently the signature to create a new custom element Transform is unintuitive, rather complex and ugly:

$this->transforms['channel'] = new Transform(
  function (DOMElement $node, Item $site) {
    // do stuff
    return $this;
  }
);

Add a wrapper to Converter to streamline these calls, i.e.

/* probable signature */
public function addTransform(string $nodeName, Closure|string $callback) { /*...*/ }

within the custom Converter

/* 1) pass an anonymous function */
$this->addTransform('language', function($node,$item) {});
/* 2) pass an object byref with a "language" method, calls $myhelper->language() */
$this->addTransform('language', $myhelper);
/* 3) pass the name of a method within the custom Converter, calls $this->langCoder() */
$this->addTransform('language', "langCoder");

Option 2 would allow to include (share) external code (plugins) to perform the same Transform job over and over again.