espanso: configure toggle key

From the configuration docs:

> If you often need to quickly enable and disable Espanso during regular
use, you might want to customize the Toggle Key. When double pressed,
the Toggle Key disables Espanso, preventing any
expansion. Double-pressing Alt again will re-enable it.

> Prior to version 2.1.2, Espanso was configured to use the ALT key as
toggle_key by default. That was a major source of confusion, as many
users accidentally pressed it during normal use. For this reason,
Espanso now ships with the toggle_key disabled by default.

https://espanso.org/docs/configuration/options/#customizing-the-toggle-key

home/opdavies/modules/espanso.nix

@@ -5,6 +5,7 @@
     configs = {
       default = {
         show_notifications = false;
+        toggle_key = "LEFT_ALT";
       };
     };