WIP: Support multiline strings

Changes corresponding to ghc!12089 (merged)

Don't merge this MR until other MR is merged. See instructions in other MR for more details.

haddock-api/src/Haddock/Backends/Hyperlinker/Parser.hs

@@ -320,6 +320,7 @@ classify tok =
 
     ITchar              {} -> TkChar
     ITstring            {} -> TkString
+    ITmultilinestring   {} -> TkString
     ITinteger           {} -> TkNumber
     ITrational          {} -> TkNumber