Write a function named add_stars that accepts as a parameter an array
of strings, and modifies the array by placing a "*" element between elements, as well as at the start
and end of the array.
For example, if the array $arr contains ["the", "quick", "brown", "fox"], the call of
add_stars($arr) should modify it to store ["*", "the", "*", "quick", "*", "brown", "*", "fox", "*"].
If the array passed is empty, you should add a single "*" to the array, resulting in ["*"].
A note about references in PHP: In order to write a function passes a parameter as
reference (thus modifying its state), you'll need to prepend "&" to the variable declaration
in the function header. For example, a function foo that modifies the state
of an array parameter may be defined as:
function foo(&$arr) { ... }
