LibMarkdown: Don't parse lines starting with a space as heading

This fixes a bug where lines starting with a space would get parsed as
"level 0" headings - it would not find a "#" and therefore never
increase the level counter (starting at zero), which then would cause
the check for "space after #" pass (again, there is no "#").

Eventually we would get funny results like this:

    <h0>[n-1 spaces]oops!</h0>

Also ASSERT(level > 0) in the Heading constructor.

Libraries/LibMarkdown/Heading.h

@@ -40,6 +40,7 @@ public:
         : m_text(move(text))
         , m_level(level)
     {
+        ASSERT(m_level > 0);
     }
     virtual ~Heading() override { }