[11.x] Add Relation::getMorphAlias()

[pxlrbt]

Since there is a Relation::getMorphedModel() I think it would be beneficial to add the reverse getMorphAlias.

This addition is particularly beneficial for testing, as it simplifies assertions with $this->assertDatabaseHas. Developers no longer need to recall the morph alias used:

$this->assertDatabaseHas('taskables', [
   'taskable_type' => Relation::getMorphAlias(Document::class),
   'taskable_id' => $mitigation->id,
   'task_id' => $taskB->id
]);

A similar PR with a broader scope was previously submitted:
#43487

[Tofandel]

Shouldn't this have been array_search($className, static::$morphMap) ?: $className

array_search performs about 3 times better in this kind of configuration than array_flip

https://onlinephp.io/c/43ff4

Why return null and not the original class name?

This makes the example in the description much more verbose now and not a drop in replacement for $model->getMorphClass()

$this->assertDatabaseHas('taskables', [
   'taskable_type' => Relation::getMorphAlias(Document::class) ?? Document::class,
   'taskable_id' => $mitigation->id,
   'task_id' => $taskB->id
]);

[calebdw]

Fixed: #51845

[pxlrbt]

Thanks @calebdw for improving this.