vim-patch:8.2.0602: :unlet $VAR does not work properly (#13238)

Problem:    :unlet $VAR does not work properly.
Solution:   Make ":lockvar $VAR" fail.  Check the "skip" flag.
7e0868efcf

Include patch 8.2.0601 changes so that ex_unletlock() can execute a callback if there are no errors.

src/nvim/eval.h

@@ -233,6 +233,8 @@ typedef enum {
   kDictListItems,  ///< List dictionary contents: [keys, values].
 } DictListType;
 
+typedef int (*ex_unletlock_callback)(lval_T *, char_u *, exarg_T *, int);
+
 // Used for checking if local variables or arguments used in a lambda.
 extern bool *eval_lavars_used;