feat(tazjin/home): add emacs-vterm integration to fish shell

Turns out there's some fancy extra features like syncing the shell and
the buffer directory, which ezemtsov discovered recently.

Change-Id: Ibb6f222d277e2cb3725387364d43e7b916b6df35
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11216
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

users/tazjin/home/shared.nix

@@ -30,6 +30,10 @@ in
   programs.fish = {
     enable = true;
     interactiveShellInit = ''
+      # emacs vterm integration
+      source (find '${pkgs.emacsPackages.vterm}' -name 'emacs-vterm.fish')
+
+      # z
       ${pkgs.zoxide}/bin/zoxide init fish | source
     '';
   };