LibMarkdown: Implement "tightness" for lists

From the commonmark spec:
A list is loose if any of its constituent list items are separated by
blank lines, or if any of its constituent list items directly contain
two block-level elements with a blank line between them. Otherwise a
list is tight. (The difference in HTML output is that paragraphs in a
loose list are wrapped in <p> tags, while paragraphs in a tight list are
not.)

Userland/Libraries/LibMarkdown/Heading.cpp

@@ -9,7 +9,7 @@
 
 namespace Markdown {
 
-String Heading::render_to_html() const
+String Heading::render_to_html(bool) const
 {
     return String::formatted("<h{}>{}</h{}>\n", m_level, m_text.render_to_html(), m_level);
 }