Shared: avoid redundant T.pack in tabFilter

Use the OverloadedStrings literal " " directly instead of
T.pack " ", avoiding an unnecessary pack call on every tab.

Co-Authored-By: Claude <noreply@anthropic.com>

src/Text/Pandoc/Shared.hs

@@ -263,7 +263,7 @@ tabFilter tabStop = T.unlines . map go . T.lines
          in  if T.null s2
                 then s1
                 else s1 <> T.replicate
-                       (tabStop - (T.length s1 `mod` tabStop)) (T.pack " ")
+                       (tabStop - (T.length s1 `mod` tabStop)) " "
                        <> go (T.drop 1 s2)
 
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