[nix-repl] Show which variables get loaded

[kstrafe]

It's not practical to run nix repl nixpkgs and only get "Added 6 variables". This does not tell me which variables. I have to run the output against a plain nix repl, perform tab completion, and then diff these two outputs.

I'd like to just print the variables which were just loaded into the global scope.

[pbsds]

should be capped

$ nix repl '<nixpkgs>'
warning: future versions of Lix will require using `--file` to load a file
Lix 2.90.0
Type :? for help.
Loading installable ''...
Added 21545 variables.

[kstrafe]

@pbsds Good point. Perhaps we should add a command for getting the most recently loaded variables and show these in a pager.

[kstrafe]

PR #11406