patch 8.2.4335: no autocommand event triggered before changing directory

Problem:    No autocommand event triggered before changing directory. (Ronnie
            Magatti)
Solution:   Add DirChangedPre. (closes #9721)
diff --git a/src/vim.h b/src/vim.h
index 67b4e33..166edf5 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1304,6 +1304,7 @@
     EVENT_CURSORMOVEDI,		// cursor was moved in Insert mode
     EVENT_DIFFUPDATED,		// after diffs were updated
     EVENT_DIRCHANGED,		// after user changed directory
+    EVENT_DIRCHANGEDPRE,	// before directory changes
     EVENT_ENCODINGCHANGED,	// after changing the 'encoding' option
     EVENT_EXITPRE,		// before exiting
     EVENT_FILEAPPENDCMD,	// append to a file using command