Lua writer: provide global PANDOC_WRITER_OPTIONS

Closes: #6731

doc/lua-filters.md

@@ -267,6 +267,10 @@ variables.
     While the object can be modified, the changes will **not**
     be picked up by pandoc.
 
+    This variable is also set in custom writers.
+
+    *Since: pandoc 2.17*
+
 `PANDOC_VERSION`
 :   Contains the pandoc version as a [Version] object which
     behaves like a numerically indexed table, most significant

src/Text/Pandoc/Writers/Custom.hs

@@ -89,6 +89,7 @@ writeCustom :: (PandocMonad m, MonadIO m)
 writeCustom luaFile opts doc@(Pandoc meta _) = do
   let globals = [ PANDOC_DOCUMENT doc
                 , PANDOC_SCRIPT_FILE luaFile
+                , PANDOC_WRITER_OPTIONS opts
                 ]
   res <- runLua $ do
     setGlobals globals