Deprecate recursive mode

The recursive mode has caused problems because it doesn't do any
filtering, which can mess with files in `.git` directories and
elsewhere. While we could support sane implicit filters and an interface
to filter explicitly, that adds complexity and maintenance burden.

Instead, we can promote the use of `treefmt` instead, a "formatting
multiplexer", which supports file filtering by default. So `nixfmt` will
only be the "backend" formatter, while `treefmt` is the frontend.

Previously discussed in a team meeting here:
#151 (comment)

main/Main.hs

@@ -92,6 +92,7 @@ collectNixFiles path = do
   dir <- doesDirectoryExist path
   if
     | dir -> do
+        hPutStrLn stderr "\ESC[33mRecursing into directories is deprecated and will be unsupported soon, please use https://treefmt.com/ instead\ESC[0m"
         files <- listDirectory path
         concat <$> mapM collectNixFiles ((path </>) <$> files)
     | ".nix" `isSuffixOf` path -> pure [path]