Enforce coding guidelines for trailing comma in multi-line array declaration

Since 20th May 2020, this rule applies:

1. The uses of a trailing comma on multi-line array declaration is required in PHP and JS.
2. It's NOT required to change all current old-style occurrences in the codebase. We'll follow a progressive approach here (new code and modified code)

For example for the expected format:

$foo = [
    'a',
    'b',
    'c',
];