Lua: restore List behavior of MetaList

Fixes a regression introduced in 2.16 which had MetaList elements loose
the `pandoc.List` properties.

Fixes #7650

data/pandoc.lua

@@ -55,6 +55,7 @@ local function create_accessor_behavior (tag)
     if k == "t" then
       return getmetatable(t)["tag"]
     end
+    return getmetatable(t)[k]
   end
   behavior.__pairs = function (t)
     return next, t

test/lua/module/pandoc.lua

@@ -278,6 +278,19 @@ return {
       end)
     },
   },
+  group 'MetaValue elements' {
+    test('MetaList elements behave like lists', function ()
+      local metalist = pandoc.MetaList{}
+      assert.are_equal(type(metalist.insert), 'function')
+      assert.are_equal(type(metalist.remove), 'function')
+    end),
+    test('MetaList, MetaMap, MetaInlines, MetaBlocks have `t` tag', function ()
+      assert.are_equal((pandoc.MetaList{}).t, 'MetaList')
+      assert.are_equal((pandoc.MetaMap{}).t, 'MetaMap')
+      assert.are_equal((pandoc.MetaInlines{}).t, 'MetaInlines')
+      assert.are_equal((pandoc.MetaBlocks{}).t, 'MetaBlocks')
+    end)
+  },
   group 'Other types' {
     group 'SimpleTable' {
       test('can access properties', function ()