Add defineBehaviors() to base Model

[bencroker]

This PR adds a defineBehaviors() to the base Model class, making it easier for models to attach behaviors without having to call the parent method, similar to defineRules(). It also uses the behaviors attribute of the DefineBehaviorsEvent event so that plugins can modify behaviors already defined.

Usage before:

    public function behaviors(): array
    {
        $behaviors = parent::behaviors();
        $behaviors['parser'] = [
            'class' => EnvAttributeParserBehavior::class,
            'attributes' => ['apiKey'],
        ];

        return $behaviors;
    }

Usage after:

    public function defineBehaviors(): array
    {
        return [
            'parser' => [
                'class' => EnvAttributeParserBehavior::class,
                'attributes' => ['apiKey'],
            ],
        ];
    }

[brandonkelly]

Like it, thanks!