Review sniff handling of PHP 7.3 trailing comma in function calls

[dingo-d]

Since PHP 7.3, adding a trailing comma in function calls, like:

array_merge( $array_one, $array_two, $array_three, );

won't throw a parse error.

This has been motivated by the fact that you could have a trailing comma in an array declaration since always:

array(
  'hello',
  'world',
);

and in grouped namespace syntax since PHP 7.2:

use Foo\Bar\{
    Foo,
    Bar,
};

There are examples of what's not allowed in the RFC.

If I'm not mistaken, PEAR.Functions.FunctionCallSignature sniff should handle it, and we are including it in the WordPress-Core ruleset. Once we up the requirement of PHPCS to 3.7.0, we should just add extra test cases to see if it's handled correctly.

Refs:

• PHP RFC: https://wiki.php.net/rfc/trailing-comma-function-calls

Related to #764

[jrfnl]

Closing as duplicate of #1363