config: add aliases for "education shell"

i have one big devshell with all the "education dependencies", i.e. the
exact right version of java, python (with deps), etc. ideally i'd have
one dev-shell per course, but i want to avoid it copying the entire
course into the nix store, hence this extra indirection of `/shell` in
the education folder.

bit of speculation, but perhaps one day it could search upwards (from
the current directory) till it finds a parent directory with a `/shell`
directory, then execute `nom develop` on that. that way i could avoid
copying everything into the nix store, but also have per-course
devshells. definitely worth a thought!

config.nix

@@ -107,6 +107,8 @@ let
         tmp = "pushd $(mktemp -d)";
         gc-nix = "${inhibit} nix-env --delete-generations +3 && nix store gc --verbose && nix store optimise --verbose";
         inhibit = "gnome-session-inhibit";
+        edushell = "nom develop $HOME/assets/education/shell";
+        edushell-offline = "${edushell} --offline";
       };
 
     functions = {