LibMarkdown: Rewrite Inline text parser to be more forgiving

The previous Text::parse was not able to give up on parsing a textual
element, and just leave it as plain text. Because this is a very
important part of markdown, I fully rewrote the parser to support this
without having to backtrack. Also the parser now some other little
features, such ast delimiter runs and flanking.

Userland/Libraries/LibMarkdown/List.cpp

@@ -59,10 +59,7 @@ OwnPtr<List> List::parse(Vector<StringView>::ConstIterator& lines)
             return true;
 
         auto text = Text::parse(item_builder.string_view());
-        if (!text.has_value())
-            return false;
-
-        items.append(move(text.value()));
+        items.append(move(text));
 
         item_builder.clear();
         return true;