pandoc.lua: set metatable on List MetaValues

The `List` metatable is assigned to the tables which get passed to the
constructors `MetaBlocks`, `MetaInline`, and `MetaList`. This enables
the use of the resulting objects as lists.  This is part of the changes
discussed in #4081.

data/pandoc.lua

@@ -185,24 +185,24 @@ end
 --- Meta list
 -- @function MetaList
 -- @tparam {MetaValue,...} meta_values list of meta values
+M.meta_value_list_types = {
+  "MetaBlocks",
+  "MetaInlines",
+  "MetaList",
+}
+for i = 1, #M.meta_value_list_types do
+  M[M.meta_value_list_types[i]] = M.MetaValue:create_constructor(
+    M.meta_value_list_types[i],
+    function(content)
+      return List:new(content)
+    end
+  )
+end
 
 --- Meta map
 -- @function MetaMap
 -- @tparam table key_value_map a string-indexed map of meta values
-M.meta_value_types = {
-  "MetaBlocks",
-  "MetaInlines",
-  "MetaList",
-  "MetaMap",
-}
-for i = 1, #M.meta_value_types do
-  M[M.meta_value_types[i]] = M.MetaValue:create_constructor(
-    M.meta_value_types[i],
-    function(content)
-      return content
-    end
-  )
-end
+M.MetaValue:create_constructor("MetaMap", function (mm) return mm end)
 
 --- Creates string to be used in meta data.
 -- Does nothing, lua strings are meta strings.