eval: Split and move dict_add_nr_str to typval.c

Function was split into tv_dict_add_nr() and tv_dict_add_str().

src/nvim/search.c

@@ -356,9 +356,10 @@ int pat_has_uppercase(char_u *pat)
   return FALSE;
 }
 
-char_u *last_csearch(void)
+const char *last_csearch(void)
+  FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  return lastc_bytes;
+  return (const char *)lastc_bytes;
 }
 
 int last_csearch_forward(void)