You can literally use a literal JSON text in both cases.

\n works in pack.mcmeta

I've never tested this out, but I believe the unicode notation is the way to go because when opening the mcmeta file, it won't allow the symbol to show up, unless you decide to screw up the file just so you can make it accept unicode symbols. But hey, I might be wrong. I haven't placed any unicode symbols in my pack's mcmeta file, so I can't really say for certain if it works (Though, you can test this out by backing up your mcmeta file and changing it to unicode just so you can add the symbol in, and if it works, good for you! If not, try the notation instead). However, the text files of the pack (splash.txt, end.txt, and credits.txt) can all be edited to have unicode symbols without the need of using any notations (Just in case you were wondering)!