use a trailing comma in arrays

While I am at it, things like:

{$a=[
  'foo',
  'bar',
  'baz',
]}

are not possible at the moment (because of the trailing comma). PHP supports this syntax for quite a while, so IMO Smarty should do it as well. IMO (but as I said, I am not really good with parsers), a small change should to the trick:

arrayelements(res)   ::=  arrayelements(a1) COMMA arrayelement(a).  {
    res = a1.','.a;
}

arrayelements(res)   ::=  arrayelement(a1) COMMA arrayelements(a).  {
    res = a1.','.a;
}

The issue identifies the arrayelements grammar productions as the entry point; start by locating the parser grammar that handles array elements and compare it with the trailing-comma PHP example. Done means the example parses successfully while existing array syntax remains valid.