Clarify coding style for trailing comma in multi-line array declaration

I'd like to propose that we recommend or require the use of a trailing comma on multi-line array declarations, in PHP and JS.

For example:

$foo = [

  'a',

  'b',

  'c',

];

Rationale:

1. Adding additional keys to the array later leads to fewer lines of diff (1 addition, vs. 1 removal + 2 addition)
2. Git blame and git history are clearer.

So this is a proposal to require the use of a trailing comma on multi-line array declarations.

Associated actions include:

a) Modify codechecker in order to provide basic support to the proposal above.
b) Document the change
c) Explicitly, 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).

And that's all. Options are:

A) We accept the change to require the use of a trailing comma on multi-line array declarations.
B) We continue as we are.