patch 9.1.0207: No autocommand when writing session file

Problem:  No autocommand when writing session file
Solution: Add SessionWritePost autocommand
          (Colin Kennedy)

fixes: ##14242
closes: #14288

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Colin Kennedy <colinvfx@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim.h b/src/vim.h
index a04be08..63a3188 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1404,6 +1404,7 @@
     EVENT_SAFESTATE,		// going to wait for a character
     EVENT_SAFESTATEAGAIN,	// still waiting for a character
     EVENT_SESSIONLOADPOST,	// after loading a session file
+    EVENT_SESSIONWRITEPOST,	// after writing a session file
     EVENT_SHELLCMDPOST,		// after ":!cmd"
     EVENT_SHELLFILTERPOST,	// after ":1,2!cmd", ":w !cmd", ":r !cmd".
     EVENT_SIGUSR1,		// after the SIGUSR1 signal