Dusk: adding ifAvailable() to compliment the existing whenAvailable()

[u01jmg3]

Laravel Dusk already provides whenAvailable(). However, I need to support (conditional) modals which may or may not appear depending on the data I have loaded in my app. For this reason, it would be useful to have ifAvailable() which behaves similar to whenAvailable() with one key difference. If the given selector does not become available after a specified amount of time, Dusk proceeds without throwing an error. If the selector does become available then ifAvailable() behaves just like whenAvailable() and calls the given callback.

Would this be helpful to have in the core of Laravel Dusk?

One caveat is it would mean having two new internal methods.

1. The first I've called waitForOrContinue() that is called in place of ->waitFor.
   • Crucially this method uses find() (rather than findOrFail()) and returns $this if the selector cannot be found. It also has no timeout logic as this is no longer relevant.
2. The second I've called withOrContinue() that is called in place of ->with.
   • All it does is return $this if the selector cannot be found otherwise it calls the existing with() method.

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Attempted PR: laravel/dusk#933