Add collection syntax support for iterable

[julienfalque]

This PR adds support for parsing iterable using the collection syntax such as iterable<int,string>.

[jaapio]

Thanks for your contribution.

Could you please explain why you want to have this syntax? This is a bc break would a new type help to solve the problem you are facing?

[julienfalque]

I need this syntax to be able to document *ToMany fields in some Doctrine entities. I don't want the entities to have any knownledge about Doctrine so instead of this:

/**
 * @var array|Collection
 */
private $items;

I prefer this:

/**
 * @var iterable
 */
private $items;

Then I add the item type to improve static analysis with e.g. PHPStan:

/**
 * @var iterable<SomeEntity>
 */
private $items;

Though this currently fails on a project that relies on this library to fetch properties metadata because the library throws an exception when trying to parse the expression.

Which part of these changes do you consider a BC break?

[jaapio]

I must have missed the default failback of the mixed type while reviewing this. Thanks this looks good. I will merge this one.

[julienfalque]

Thank you :)