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/autocmd.c b/src/autocmd.c
index 8e43b34..8a7e607 100644
--- a/src/autocmd.c
+++ b/src/autocmd.c
@@ -160,6 +160,7 @@
     {"SafeState",	EVENT_SAFESTATE},
     {"SafeStateAgain",	EVENT_SAFESTATEAGAIN},
     {"SessionLoadPost",	EVENT_SESSIONLOADPOST},
+    {"SessionWritePost",EVENT_SESSIONWRITEPOST},
     {"ShellCmdPost",	EVENT_SHELLCMDPOST},
     {"ShellFilterPost",	EVENT_SHELLFILTERPOST},
     {"SigUSR1",		EVENT_SIGUSR1},