[5.7] Add orWhere proxy for scopes to Eloquent Builder

[erikgaal]

Overview

This PR adds a shorter way of combining Model query scopes using the same technique applied in Laravel's Collections. Currently, combining model scopes with an OR operator adds more complexity in than necessary.

// scopeFoo, scopeBar, scopeBaz on the Model class 
$query->foo()->orWhere(function (Builder $query) {
    $query->bar();
})->orWhere(function (Builder $query) {
    $query->baz();
});

This PR allows chaining of OR's without the Closure in user code, like so.

$query->foo()->orWhere->bar()->orWhere->baz();

It follows the same logic as combining regular where and orWhere. The following code examples are thus considered equivalent.

// scopeFoo, scopeBar, scopeBaz on the Model class 
$query->foo()->orWhere(function (Builder $query) {
    $query->bar();
})->baz();

$query->foo()->orWhere->bar()->baz();

Considerations

1. This PR does not break any existing functionality as far as I know.

2. I have considered another approach to achieve the same simplicity. It would add the possibility to call query scopes prepended with or, like so. But to my knowledge, this seemed more confusing than the proposed change.

$query->foo()->orBar()->orBaz();