Add assertion to check if a button is disabled

[SjorsO]

This PR adds the following two assertions:

public function assertButtonEnabled($button)

public function assertButtonDisabled($button)

Asserting that a button is disabled is something I frequently do, I feel like it deserves its own assertion. Both new assertions select a button in the same way ->press($button) does.

Without these assertions, checking if a button is disabled has to be done something like this:

->tap(function (Browser $browser) {
    $this->assertFalse($browser->resolver->resolveForButtonPress('Press me')->isEnabled());
})

There is already a assertDisabled($field) assertion, but since that uses ->resolveForField() it can not be used to select buttons.

[staudenmeir]

I think assertButtonEnabled() and assertButtonDisabled() would be more consistent with the existing assertions.

[SjorsO]

Good point, I've renamed them