Support optional comma in closure use list

RFC: https://wiki.php.net/rfc/trailing_comma_in_closure_use_list
Discussion: https://externals.io/message/110715

The release manager has agreed to allow merging of RFCs that have near-unanimous
votes. If an RFC ends up not achieving the required 2/3 majority at the time the
announced voting period closes, this implementation commit will be reverted
in time for the feature freeze.

Closes GH-5793

Zend/tests/closure_use_trailing_comma.phpt

@@ -0,0 +1,17 @@
+--TEST--
+Closure use list can have trailing commas
+--FILE--
+<?php
+
+$b = 'test';
+$fn = function () use (
+    $b,
+    &$a,
+) {
+    $a = $b;
+};
+$fn();
+echo "$a\n";
+?>
+--EXPECT--
+test

Zend/zend_language_parser.y

@@ -1155,7 +1155,7 @@ returns_ref:
 
 lexical_vars:
 		%empty { $$ = NULL; }
-	|	T_USE '(' lexical_var_list ')' { $$ = $3; }
+	|	T_USE '(' lexical_var_list possible_comma ')' { $$ = $3; }
 ;
 
 lexical_var_list: