Rename t to tmux-sessionizer

This matches ThePrimeagen's script that mine is based on and that I'll
probably switch to in the future.

I've added to the Nix store as custom-tmux-sessionizer so it doesn't
conflict with the tmux-sessionzer packages that's already in the store.

nix/modules/home-manager/features/cli/default.nix

@@ -19,6 +19,15 @@
         ;
     })
 
+    (import ./tmux-sessionizer.nix {
+      inherit
+        config
+        lib
+        pkgs
+        self
+        ;
+    })
+
     ./bat.nix
     ./bin.nix
     ./bluetuith.nix