Parser: Add "void" block signature

Sometimes a block doesn't need anything inside of the block comment tags
as children. One example of this is the traditional "more tag," which
only needs to indicate position.

These changes introduce a new syntax element, the "void block,"
borrowing from the HTML5 nomenclature for HTML void elements.

In this new element, there are no children.

Since nothing changes on the JavaScript size this could be considered an
optimization whereby any block with no children serializes to a
self-closing block comment tag, saving some space and improving
readability of the `post_content`.

blocks/api/post.pegjs

@@ -15,9 +15,18 @@ WP_Block_List
   = WP_Block*
 
 WP_Block
-  = WP_Block_Balanced
+  = WP_Block_Void
+  / WP_Block_Balanced
   / WP_Block_Html
 
+WP_Block_Void
+  = "<!--" __ "wp:" blockType:WP_Block_Type attrs:HTML_Attribute_List _? "/-->"
+  { return {
+    blockType: blockType,
+    attrs: attrs,
+    rawContent: ''
+  } }
+
 WP_Block_Balanced
   = s:WP_Block_Start ts:(!WP_Block_End c:Any { return c })* e:WP_Block_End & { return s.blockType === e.blockType }
   { return {

post-content.js

@@ -16,6 +16,8 @@ window._wpGutenbergPost = {
 			'<p style="text-align:right;">... like this one, which is separate from the above and right aligned.</p>',
 			'<!-- /wp:core/text -->',
 
+			'<!-- wp:core/more /-->',
+
 			'<!-- wp:core/text -->',
 			'<p>Headings are separate blocks as well, which helps with the outline and organization of your content.</p>',
 			'<!-- /wp:core/text -->',