refactor: change some xstrndup() and xstrnsave() to xmemdupz() (#25959)

When the given length is exactly the number of bytes to copy, xmemdupz()
makes the intention clearer.

src/nvim/api/private/helpers.h

@@ -75,8 +75,6 @@
 #define PUT_C(dict, k, v) \
   kv_push_c(dict, ((KeyValuePair) { .key = cstr_as_string(k), .value = v }))
 
-#define PUT_BOOL(dict, name, condition) PUT(dict, name, BOOLEAN_OBJ(condition));
-
 #define ADD(array, item) \
   kv_push(array, item)