Fix add-handler for new routes file path

https://github.com/yesodweb/yesod/pull/1686 recommended `.yesodroutes` as the file extension for the routes file, and https://github.com/yesodweb/yesod-scaffold/pull/203 added that to the scaffolding. But yesod-bin assumes the file is just named `routes`.

This PR will check if the old path exists and use that, and if not use the new path.

I tested this on the yesod-scaffold repo with both the old and new filename

yesod-bin/AddHandler.hs

@@ -84,13 +84,22 @@ addHandlerInteractive = do
     methods <- getLine
     addHandlerFiles cabal routePair pattern methods
 
+getRoutesFilePath :: IO FilePath
+getRoutesFilePath = do
+    let oldPath = "config/routes"
+    oldExists <- doesFileExist oldPath
+    pure $ if oldExists
+        then oldPath
+        else "config/routes.yesodroutes"
+
 addHandlerFiles :: FilePath -> (String, FilePath) -> String -> String -> IO ()
 addHandlerFiles cabal (name, handlerFile) pattern methods = do
     src <- getSrcDir cabal
     let applicationFile = concat [src, "/Application.hs"]
     modify applicationFile $ fixApp name
     modify cabal $ fixCabal name
-    modify "config/routes" $ fixRoutes name pattern methods
+    routesPath <- getRoutesFilePath
+    modify routesPath $ fixRoutes name pattern methods
     writeFile handlerFile $ mkHandler name pattern methods
     specExists <- doesFileExist specFile
     unless specExists $