Improve relation return types

[SanderMuller]

Relations currently signal that they return the following: @return \Staudenmeir\LaravelAdjacencyList\Eloquent\Relations\belongsToManyOfDescendants<static>

While they don't create a relation query builder of static (the declaring model) but actually for the related model as passed as class string to the method. For example in User this code snipped:

    public function roles(): BelongsToManyOfDescendants
    {
        return $this->belongsToManyOfDescendants(Role::class);
    }

Currently the return type would say that User would be the query target, while it's actually Role. This PR adds the generic TRelatedModel of \Illuminate\Database\Eloquent\Model to these methods and updates @param string $related to @param class-string<TRelatedModel> $related so we can now define the return type as @return \Staudenmeir\LaravelAdjacencyList\Eloquent\Relations\HasManyOfDescendants<TRelatedModel> correctly signalling that we are creating a query of the passed class string of TRelatedModel

[staudenmeir]

Thanks!