LibMarkdown: Change internal MD API to return OwnPtrs

Previously, all Markdown blocks had a virtual parse method which has
been swapped out for a static parse method returning an OwnPtr of
that block's type.

The Text class also now has a static parse method that will return an
Optional<Text>.

Userland/md.cpp

@@ -25,6 +25,7 @@
  */
 
 #include <AK/ByteBuffer.h>
+#include <AK/OwnPtr.h>
 #include <AK/String.h>
 #include <LibCore/File.h>
 #include <LibMarkdown/Document.h>