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/eval.h

@@ -134,6 +134,7 @@ typedef enum {
     VV_TYPE_DICT,
     VV_TYPE_FLOAT,
     VV_TYPE_BOOL,
+    VV_EXITING,
 } VimVarIndex;
 
 /// All recognized msgpack types