gfm/commonmark writer: implement start number on ordered lists.

Previously they always started at 1, but according to the spec
the start number is respected. Closes #7009.

src/Text/Pandoc/Writers/Markdown.hs

@@ -674,7 +674,10 @@ blockToMarkdown' opts (BulletList items) = do
   contents <- inList $ mapM (bulletListItemToMarkdown opts) items
   return $ (if isTightList items then vcat else vsep) contents <> blankline
 blockToMarkdown' opts (OrderedList (start,sty,delim) items) = do
-  let start' = if isEnabled Ext_startnum opts then start else 1
+  variant <- asks envVariant
+  let start' = if variant == Commonmark || isEnabled Ext_startnum opts
+                  then start
+                  else 1
   let sty'   = if isEnabled Ext_fancy_lists opts then sty else DefaultStyle
   let delim' = if isEnabled Ext_fancy_lists opts then delim else DefaultDelim
   let attribs = (start', sty', delim')

test/command/7009.md

@@ -0,0 +1,8 @@
+```
+% pandoc -t gfm
+3. a
+4. b
+^D
+3.  a
+4.  b
+```