patch 8.2.2070: can't get the exit value in VimLeave(Pre) autocommands

Problem:    Can't get the exit value in VimLeave or VimLeavePre autocommands.
Solution:   Add v:exiting like in Neovim. (Yegappan Lakshmanan, closes #7395)
diff --git a/src/vim.h b/src/vim.h
index 6dcc509..eda0734 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1996,7 +1996,8 @@
 #define VV_ECHOSPACE	93
 #define VV_ARGV		94
 #define VV_COLLATE      95
-#define VV_LEN		96	// number of v: vars
+#define VV_EXITING	96
+#define VV_LEN		97	// number of v: vars
 
 // used for v_number in VAR_BOOL and VAR_SPECIAL
 #define VVAL_FALSE	0L	// VAR_BOOL