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.

kitty/child.py

@@ -152,6 +152,7 @@ def process_env(env: Mapping[str, str] | None = None) -> dict[str, str]:
         ans.pop(ssl_env_var, None)
     ans.pop('XDG_ACTIVATION_TOKEN', None)
     ans.pop('VTE_VERSION', None)  # Used by the stupid VTE shell integration script that is installed system wide, sigh
+    ans.pop('KITTY_SI_RUN_COMMAND_AT_STARTUP', None)
     return ans