Lua: add new function pandoc.types.Sources.

This provides access to the `Sources` abstraction that pandoc uses for
input files.

Closes: #11441

tools/update-lua-module-docs.lua

@@ -100,6 +100,8 @@ local known_types = {
   ReaderOptions = 'type-readeroptions',
   Row = 'type-row',
   SimpleTable = 'type-simpletable',
+  Source = 'type-pandoc.types.Source',
+  Sources = 'pandoc.types.Sources',
   Span = 'type-span',
   Str = 'type-str',
   Table = 'type-table',