On Mac, Homebrew (brew) will allow you to easily upgrade nano to a newer version than the one that came with Mac OSX.
Install brew, then install a new version of nano from the Terminal.
brew install nano
Installing this way includes the /usr/local/share/nano folder containing the default syntax highlight files. See also the extra subdirectory.
To enable all default syntaxes, add a wildcard include to your ~/.nanorc.
include "/usr/local/share/nano/*.nanorc"
For Apple Silicon (M1), the path is under /opt/homebrew/:
include "/opt/homebrew/share/nano/*.nanorc"