Enable InMemory repository to accept items in constructor

[Bl00D4NGEL]

Currently the InMemoryRepository has no way to accept any items making it basically a "NullRepository". This PR adds the items parameeter to the constructor (as a promoted property) to help mitigate this.

NOTE: This should be merged after #18

Previously a class that extends the Repository would write something like this:

class FooRepository extends Repository {
    public function __construct() {
        parent::__construct(Foo::class);
        $this->items = [new Foo()];
    }
}

If this PR gets merged a user could use a more straight forward interface:

class FooRepository extends Repository {
    public function __construct() {
        parent::__construct(Foo::class, [new Foo()]);
    }
}

In addition this enables users of the InMemoryRepository to implement the "filter" option a bit more straight forward.

Instead of this

public function findByEmail(string $email): self
{
    $filtered = [];
    foreach ($this->items as $item) {
        if ($item->email === $email) {
            filtered[] = $item;
        }
    }

    $self = new self($this->itemType);
    $self->items = $filtered;

     return $self;
}

A user can now do this:

public function findByEmail(string $email): self
{
    $filtered = [];
    foreach ($this->items as $item) {
        if ($item->email === $email) {
            filtered[] = $item;
        }
    }

    return new self($this->itemType, $filtered);
}