Add an env var that can be used to eval an expression at startup of shell

This will come in handy to implement serialization as session
with running of current foreground command.

shell-integration/fish/vendor_conf.d/kitty-shell-integration.fish

@@ -37,6 +37,8 @@ function __ksi_schedule --on-event fish_prompt -d "Setup kitty integration after
         end
         set --erase SSH_KITTEN_KITTY_DIR
     end
+    set --local krcs "$KITTY_SI_RUN_COMMAND_AT_STARTUP"
+    set --erase KITTY_SI_RUN_COMMAND_AT_STARTUP
 
     # Enable cursor shape changes for default mode and vi mode
     if not contains "no-cursor" $_ksi
@@ -201,6 +203,9 @@ function __ksi_schedule --on-event fish_prompt -d "Setup kitty integration after
         test (count $new_path) -eq (count $PATH)
         or set --global --export --path PATH $new_path
     end
+    if test -n "$krcs"
+        eval "$krcs"
+    end
 end
 
 function edit-in-kitty --wraps "kitten edit-in-kitty" -d "Edit the specified file in a kitty overlay window with your locally installed editor"