Lua filters: allow filtering of element lists (#6040)

Lists of Inline and Block elements can now be filtered via `Inlines` and
`Blocks` functions, respectively. This is helpful if a filter conversion
depends on the order of elements rather than a single element.

For example, the following filter can be used to remove all spaces
before a citation:

    function isSpaceBeforeCite (spc, cite)
      return spc and spc.t == 'Space'
       and cite and cite.t == 'Cite'
    end

    function Inlines (inlines)
      for i = #inlines-1,1,-1 do
        if isSpaceBeforeCite(inlines[i], inlines[i+1]) then
          inlines:remove(i)
        end
      end
      return inlines
    end

Closes: #6038

test/lua/meta.lua

@@ -0,0 +1,6 @@
+function Meta (meta)
+  meta.old = nil
+  meta.new = "new"
+  meta.bool = (meta.bool == false)
+  return meta
+end