Collections: Add copy() and clone() methods

[aimeos]

Not all collection methods return a new collection instance and some are destructive or additive.
To prevent modifying collections that are used in later code too when only a temporary copy is needed for the current operation, a copy() and clone() method would be very helpful:

foreach ($collection->copy()->push('new')->reverse() as $item)

// or for cloning objects in collections
foreach ($collection->clone()->push('new')->reverse() as $item)

Using PHP clone keyword doesn't solve the problem easily because a relation can also return NULL instead of a collection. When using clone $model->relation on NULL, a fatal error will be the result. Instead, we can use:

foreach ($model->relation?->copy()?->push('new')?->reverse() ?? [] as $item)

// or for cloning objects in collections
foreach ($model->relation?->clone()?->push(new \stdClass)?->reverse() ?? [] as $item)

copy() creates only a new collection object while clone() clones all objects within the collection too.
The PHP Map package implements them too:

• https://php-map.org/#clone
• https://php-map.org/#copy