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.

docs/glossary.rst

@@ -224,6 +224,14 @@ Variables that kitty sets when running child programs
    Set when enabling :ref:`shell_integration`. It is automatically removed by
    the shell integration scripts.
 
+.. envvar:: KITTY_SI_RUN_COMMAND_AT_STARTUP
+
+   Set this to an expression that the kitty shell integration scripts will
+   ``eval`` after the shell is started. Note that this environment variable
+   is ignored when present in the environment in which kitty itself is launched
+   in. It is most useful with the ``--env`` flag for the :doc:`launch <launch>`
+   action.
+
 .. envvar:: ZDOTDIR
 
    Set when enabling :ref:`shell_integration` with :program:`zsh`, allowing