Parser: Parse <!-- more --> tag and <!-- noteaser --> (#1460)

* Parser: Parse <!-- more --> tag and <!-- noteaser -->

These tags are officially supported by WordPress and are an exception
with the concept of blocks. They contain structural data and yet they
are just HTML comments without any further indication.

The "more" tag supports an optional custom text string and if it is
immediately followed by the "no teaser" tag then that contains
additional information.

In this change we parse those tags and combine them into a single "more"
block with attributes of the custom text and whether or not the
`noTeaser` option has been selected.

blocks/api/post.pegjs

@@ -22,10 +22,33 @@ WP_Block_List
   = WP_Block*
 
 WP_Block
-  = WP_Block_Void
+  = WP_Tag_More
+  / WP_Block_Void
   / WP_Block_Balanced
   / WP_Block_Html
 
+WP_Tag_More
+  = "<!--" WS* "more" customText:(WS+ text:$((!(WS* "-->") .)+) { /** <?php return $text; ?> **/ return text })? WS* "-->" noTeaser:(WS* "<!--noteaser-->")?
+  { /** <?php
+    return array(
+       'blockName' => 'wp:core/more',
+       'attrs' => array(
+         'customText' => $customText,
+         'noTeaser' => (bool) $noTeaser
+       ),
+       'rawContent' => ''
+    );
+    ?> **/
+    return {
+      blockName: 'wp:core/more',
+      attrs: {
+        customText: customText,
+        noTeaser: !! noTeaser
+      },
+      rawContent: ''
+    }
+  }
+
 WP_Block_Void
   = "<!--" WS+ "wp:" blockName:WP_Block_Name WS+ attrs:(a:WP_Block_Attributes WS+ {
     /** <?php return $a; ?> **/

blocks/api/serializer.js

@@ -115,6 +115,10 @@ export function serializeBlock( block ) {
 	const saveContent = getSaveContent( blockType, block.attributes );
 	const saveAttributes = getCommentAttributes( block.attributes, parseBlockAttributes( saveContent, blockType ) );
 
+	if ( 'wp:core/more' === blockName ) {
+		return `<!-- more ${ saveAttributes.customText ? `${ saveAttributes.customText } ` : '' }-->${ saveAttributes.noTeaser ? '\n<!--noteaser-->' : '' }`;
+	}
+
 	const serializedAttributes = ! isEmpty( saveAttributes )
 		? serializeAttributes( saveAttributes ) + ' '
 		: '';