[9.x] Ignore whitespaces/newlines when finding relations in model:show command

[binotaliu]

The show:model relies on reading methods' source code to finding relations. It's done by finding if the pattern \$this->{$possibleRelationMethod}( appears in the method.
In most situation, it would work:

public function user()
{
    return $this->belongsTo(User::class);
}

But if there are whitespaces/newlines after $this, the command would failed to detect:

public function user()
{
    return $this
        ->belongsTo(User::class)
        ->withDefault([
            'name' => 'Guest Author',
        ]);
}

¹

This PR fixes this issue by trimming each line of the relation method's source code, so the pattern \$this->{$possibleRelationMethod}( can be correctly detected.

public function user(){return $this->belongsTo(User::class)->withDefault(['name' => 'Guest Author',]);}

Footnotes

1. Example code from Laravel Documentation ↩