Add data traits and interfaces

[giuscris]

This PR changes how Formwork handles data classes, using traits and interfaces to add and enforce behavior.

All the new traits enforce a $data as:

protected array $data = [];

New interfaces

• Arrayable used in classes that export data to arrays
• ArraySerializable enforces a static method fromArray() to generate class instances, will be used in future in Formwork\Parsers\PHP

New traits

• DataArrayable implements toArray() method returning $data as an array, required by Arrayable
• DataCountable implements a count() method returning the count of $data, required by \Countable interface (see https://www.php.net/manual/en/class.countable.php)
• DataIterator implements an iterator on $data elements, required by \Iterator interface (see https://www.php.net/manual/en/class.iterator.php)
• DataCountableIterator is a composition of DataCountable and DataIterator
• DataGetter implements has() and get() methods
• DataSetter implements set() and remove() methods
• DataMultipleGetter implements hasMultiple() and getMultiple() extending DataGetter
• DataMultipleSetter implements setMultiple() and removeMultiple() extending DataSetter