Add assertCount() method

[ktsm-th]

This PR adds a new helper to assert the number of matching elements on the page.

It's useful when assertions are performed on DOM elements that are tied to factories:

Books::factory()
    ->times(3)
    ->sequence(['stock' => true], ['stock' => true], ['stock' => false])
    ->create();

$browser->assertCount('input[name="books_in_stock"]:checked', 2);

[github-actions]

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