Add v:exiting

Contains the exit value nvim will use.

Before exiting, it is v:null. That way jobs or autocmds (in VimLeavePre or
VimLeave) can check if Neovim is about to quit and with what exit value.

Closes #4666.

src/nvim/main.c

@@ -557,6 +557,8 @@ void getout(int exitval)
   if (exmode_active)
     exitval += ex_exitval;
 
+  set_vim_var_nr(VV_EXITING, exitval);
+
   /* Position the cursor on the last screen line, below all the text */
   ui_cursor_goto((int)Rows - 1, 0);