pandoc-quotes: use names to access Quoted element properties

Pandoc 2.15 changed the marshalling method; the undocumented `.c`
field no longer gives access to all element properties.

Author: tarleb

pandoc-quotes.lua/pandoc-quotes.lua

@@ -460,8 +460,8 @@ do
         --  the content of `quoted`, and the closing quote (as `pandoc.Str`).
         function insert_quot_marks (quoted)
             if not QUOT_MARKS then return end
-            local quote_type = quoted.c[1]
-            local inlines    = quoted.c[2]
+            local quote_type = quoted.quotetype
+            local inlines    = quoted.content
             local left, right
             if     quote_type == 'DoubleQuote' then left, right = 1, 2
             elseif quote_type == 'SingleQuote' then left, right = 3, 4