PDF: allow custom writer as format if engine is explicitly specified (#7901)

Closes #7898.
Note that it may be necessary to explicitly specify a template on the command line.

src/Text/Pandoc/App/OutputSettings.hs

@@ -254,6 +254,10 @@ pdfWriterAndProg mWriter mEngine =
       go Nothing Nothing       = Right ("latex", "pdflatex")
       go (Just writer) Nothing = (writer,) <$> engineForWriter writer
       go Nothing (Just engine) = (,engine) <$> writerForEngine (takeBaseName engine)
+      go (Just writer) (Just engine) | isCustomWriter writer =
+           -- custom writers can produce any format, so assume the user knows
+           -- what they are doing.
+           Right (writer, engine)
       go (Just writer) (Just engine) =
            case find (== (baseWriterName writer, takeBaseName engine)) engines of
                 Just _  -> Right (writer, engine)
@@ -271,6 +275,8 @@ pdfWriterAndProg mWriter mEngine =
                                 []      -> Left $
                                    "cannot produce pdf output from " <> w
 
+      isCustomWriter w = ".lua" `T.isSuffixOf` w
+
 isTextFormat :: T.Text -> Bool
 isTextFormat s =
   s `notElem` ["odt","docx","epub2","epub3","epub","pptx","pdf"]