LibMarkdown: Recognize and handle comments

This also improves Commonmark coverage, e.g. it fixes tests
HTML_blocks_ex179_2894..2906 and Lists_ex308_5439..5457.

In other words, we go from 271 out of 652 to 273 out of 652.

Userland/Libraries/LibMarkdown/CMakeLists.txt

@@ -1,6 +1,7 @@
 set(SOURCES
     BlockQuote.cpp
     CodeBlock.cpp
+    CommentBlock.cpp
     ContainerBlock.cpp
     Document.cpp
     Heading.cpp