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>.

Libraries/LibMarkdown/Block.h

@@ -37,7 +37,6 @@ public:
 
     virtual String render_to_html() const = 0;
     virtual String render_for_terminal() const = 0;
-    virtual bool parse(Vector<StringView>::ConstIterator& lines) = 0;
 };
 
 }