[6.x] Add assertInputPresent and assertInputMissing assertions

[SjorsO]

This PR adds two assertions: assertInputPresent and assertInputMissing. These assertions use the name of an input to assert if the input is present.

These assertions are useful when working with dynamic forms. I often use Livewire to hide input fields based on the value of a radio button. There is currently no convenient way to assert that an input with a specific name is missing. I'd have to add a dusk="" or id="" attribute to the input and then use assertMissing(). With these new assertions I can assert if an input is missing using its name, making extra attribute necessary.

Something to note: the name of the assertions might be slightly misleading. They don't just check input elements, but also textarea and select elements.

I haven't added any tests. I couldn't figure out how to craft a mock that calls itself. This probably wouldn't add much value to the test suite anyway. The new methods use the already tested assertPresent and assertMissing methods.