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/evalvars.c b/src/evalvars.c
index 75893bb..dea56be 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -146,6 +146,7 @@
     {VV_NAME("echospace",	 VAR_NUMBER), VV_RO},
     {VV_NAME("argv",		 VAR_LIST), VV_RO},
     {VV_NAME("collate",		 VAR_STRING), VV_RO},
+    {VV_NAME("exiting",		 VAR_SPECIAL), VV_RO},
 };
 
 // shorthand
@@ -218,6 +219,7 @@
 
     set_vim_var_nr(VV_SEARCHFORWARD, 1L);
     set_vim_var_nr(VV_HLSEARCH, 1L);
+    set_vim_var_nr(VV_EXITING, VVAL_NULL);
     set_vim_var_dict(VV_COMPLETED_ITEM, dict_alloc_lock(VAR_FIXED));
     set_vim_var_list(VV_ERRORS, list_alloc());
     set_vim_var_dict(VV_EVENT, dict_alloc_lock(VAR_FIXED));