switch vim flavour to Neovim; reason: macOS clipboard support

access to system clipboard was broken in command-line vim, the buffers
"+ or "* were said not to exist and neither unnamed or unnamedplus
clipboard settings were working.

Aliases nvim to "vim"

home/home.nix

@@ -440,8 +440,9 @@ in
       ipb = "ip --color --brief";
       ll = "ls -l";
       wavesynth = "nix-shell -p sox --run 'play -n synth brownnoise synth pinknoise mix synth 0 0 0 15 40 80 trapezium amod 0.2 20'";
+      vim = "nvim";
     };
     home.sessionVariables = {
-      EDITOR = "vim";
+      EDITOR = "nvim";
     };
 }