[12.x] Add enum functionality to morph map

[shaedrich]

Enums are a powerful addition to the PHP language and with the enum_value() helper, working with them in Laravel is like a charm. This PR adds the ability to pass an enum to another of Laravel's features: Morph maps. So, instead of a non-exhaustive array, we can now add an enumerated mapping and benefit from better IDE support.

Examples

Define morph map

// Current functionality
Relation::morphMap([ 'App\Models\Post', 'App\Models\Video' ]); // results in ['posts' => 'App\Models\Post', 'videos' => 'App\Models\Video']
Relation::morphMap([ 'articles' => 'App\Models\Post', 'clips' => 'App\Models\Video' ]); // results in ['articles' => 'App\Models\Post', 'clips' => 'App\Models\Video']

// 🆕 functionality
enum MorphMap: string
{
    case Articles = 'App\Models\Post';
    case Clips = 'App\Models\Video';
}
Relation::morphMap(MorphMap::class); // results in MorphMap{Articles: 'App\Models\Post', Clips: 'App\Models\Video'}

Get morph alias

enum MorphMap: string
{
    case Articles = 'App\Models\Post';
    case Clips = 'App\Models\Video';
}
echo Relation::getMorphAlias(MorphMap::Articles); // results in 'Articles' (a little nonsensical)
echo Relation::getMorphAlias('App\Models\Post'); // results in 'Articles'

Get morphed model

enum MorphMap: string
{
    case Articles = 'App\Models\Post';
    case Clips = 'App\Models\Video';
}
echo Relation::getMorphedModel('Articles'); // results in App\Models\Post

[shaedrich]

I'm not sure why static analysis / Types (pull_request) failed. I can't seem to find how this is related to my changes.

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If applicable, please consider releasing your code as a package so that the community can still take advantage of your contributions!