What you’re looking for is called “language injection”.
For neovim you can install the nvim-treesitter plugin with the Nix parser (and more parsers for the language you wish to highlight). If you use the unstable channel nvim-treesitter should have some of these injections by default: for example it should highlight writeScript and buildPhase as bash, etc.
I’m currently using plugin = nvim-treesitter.withAllGrammars; # Syntax Highlighting in my config and I am also on unstable. Is there something else that I need to add? I also tried adding /* lua */ in front of my multiline strings but that didn’t work either.