ParsoidExtensionAPI: Add method to process HTML embedded in attributes

* Parsoid core cannot know anything about how extensions represent
  information in their DOM. We were missing an extension API method
  and config that lets extensions specify this information along with
  handlers to process embedded content.

* This patch fills that above gap.

* There is a FIXME in ContentUtils::shiftDSR which will be addressed
  in followup patches with this new functionality.

Change-Id: If921ca471f25f21671a60c3f796fdf145267364d

src/Parsoid/Cite.php

@@ -22,7 +22,10 @@ class Cite implements ExtensionModule {
 					'name' => 'ref',
 					'handler' => Ref::class,
 					'options' => [
-						'wt2html' => [ 'unpackOutput' => false ],
+						'wt2html' => [
+							'unpackOutput' => false,
+							'embedsHTMLInAttributes' => true
+						],
 						'outputHasCoreMwDomSpecMarkup' => true
 					],
 				],
@@ -30,6 +33,9 @@ class Cite implements ExtensionModule {
 					'name' => 'references',
 					'handler' => References::class,
 					'options' => [
+						'wt2html' => [
+							'embedsHTMLInAttributes' => true
+						],
 						'html2wt' => [ 'format' => 'block' ],
 						'outputHasCoreMwDomSpecMarkup' => true
 					],

src/Parsoid/Ref.php

@@ -3,6 +3,7 @@ declare( strict_types = 1 );
 
 namespace Wikimedia\Parsoid\Ext\Cite;
 
+use Closure;
 use Exception;
 use Wikimedia\Parsoid\DOM\DocumentFragment;
 use Wikimedia\Parsoid\DOM\Element;
@@ -54,6 +55,16 @@ class Ref extends ExtensionTagHandler {
 		);
 	}
 
+	/** @inheritDoc */
+	public function processAttributeEmbeddedHTML(
+		ParsoidExtensionAPI $extApi, Element $elt, Closure $proc
+	): void {
+		$dataMw = DOMDataUtils::getDataMw( $elt );
+		if ( isset( $dataMw->body->html ) ) {
+			$dataMw->body->html = $proc( $dataMw->body->html );
+		}
+	}
+
 	/** @inheritDoc */
 	public function lintHandler(
 		ParsoidExtensionAPI $extApi, Element $ref, callable $defaultHandler

src/Parsoid/References.php

@@ -3,6 +3,7 @@ declare( strict_types = 1 );
 
 namespace Wikimedia\Parsoid\Ext\Cite;
 
+use Closure;
 use stdClass;
 use Wikimedia\Parsoid\Core\DomSourceRange;
 use Wikimedia\Parsoid\DOM\DocumentFragment;
@@ -795,6 +796,16 @@ class References extends ExtensionTagHandler {
 		return $domFragment;
 	}
 
+	/** @inheritDoc */
+	public function processAttributeEmbeddedHTML(
+		ParsoidExtensionAPI $extApi, Element $elt, Closure $proc
+	): void {
+		$dataMw = DOMDataUtils::getDataMw( $elt );
+		if ( isset( $dataMw->body->html ) ) {
+			$dataMw->body->html = $proc( $dataMw->body->html );
+		}
+	}
+
 	/** @inheritDoc */
 	public function domToWikitext(
 		ParsoidExtensionAPI $extApi, Element $node, bool $wrapperUnmodified