Allow to limit interactions within a scope

[mateusjunges]

This PR is another stab at #155, and it adds a new within() method to the Webpage class that allows scoping browser interactions to specific CSS selectors.

$page->within('#contact', function ($browser) {
    $browser->click('Submit');
});

More details

I see that the first PR got reverted because it made the Webpage non-readonly. I copied the tests from my previous PR and reworked the implementation.

I changed the return type on the InteractWithElements, MakesElementAssertions and MakesConsoleAssertions from Webpage to self, as this those traits are now also used on the ScopedWepage class.

The new ScopedWebpage now keeps context of the scope, so no changes to the Webpage are required.

[mateusjunges]

Sad to see this sitting idle for 2 months without any review 🙁