Truncate a string (first argument) if it is longer than the given maximum string length (second argument). Return the truncated string with a `...` ending.
# --hints--
`truncateString("A-tisket a-tasket A green and yellow basket", 8)` should return "A-tisket...".
```js
assert(
truncateString('A-tisket a-tasket A green and yellow basket', 8) ===
`truncateString("A-tisket a-tasket A green and yellow basket", "A-tisket a-tasket A green and yellow basket".length)` should return "A-tisket a-tasket A green and yellow basket".
`truncateString("A-tisket a-tasket A green and yellow basket", "A-tisket a-tasket A green and yellow basket".length + 2)` should return "A-tisket a-tasket A green and yellow basket".
```js
assert(
truncateString(
'A-tisket a-tasket A green and yellow basket',
'A-tisket a-tasket A green and yellow basket'.length + 2
) === 'A-tisket a-tasket A green and yellow basket'