Text.Pandoc.Lua: support Inline and Block catch-alls

Try function `Inline`/`Block` if no other filter function of the
respective type matches an element.

Closes: #3859

test/lua/block-count.lua

@@ -0,0 +1,11 @@
+local num_blocks = 0
+
+function Block(el)
+  num_blocks = num_blocks + 1
+end
+
+function Pandoc(blocks, meta)
+  return pandoc.Pandoc {
+    pandoc.Para{pandoc.Str(num_blocks)}
+  }
+end