Add basic lua List module (#4099)

The List module is automatically loaded, but not assigned to a global
variable. It can be included in filters by calling `List = require
'List'`.

Lists of blocks, lists of inlines, and lists of classes are now given
`List` as a metatable, making working with them more convenient. E.g.,
it is now possible to concatenate lists of inlines using Lua's
concatenation operator `..` (requires at least one of the operants to
have `List` as a metatable):

    function Emph (emph)
      local s = {pandoc.Space(), pandoc.Str 'emphasized'}
      return pandoc.Span(emph.content .. s)
    end

Closes: #4081

0 files changed, 0 insertions, 0 deletions