Merge previously defined view data when building the view mailable data

[tobytwigger]

Hi! 👋

In a project I manage, in which we use Telescope, we also have set up functionality to record any sent emails in the database using the Illuminate\Mail\Mailable::buildViewDataUsing() callback to pass the payload to the MessageSending or MessageSent events.

When we have Telescope enabled, we find that this functionality breaks. This is because when Telescope starts it registers its own data to build up views with, and therefore overwrites our mailable data. For reference, this is where we define our own mailable data: https://github.com/bristol-su/portal-mail/blob/develop/src/MailServiceProvider.php#L89

This PR retrieves the current callback during the start function, and then if it isn't empty calls it when building View data and merges the result with Telescopes view data. This won't lead to any breaking changes since all the Telescope data is still present and prioritised over existing data, but will allow telescope users to still use the overriding view data feature.

[taylorotwell]

Thanks for contributing to Laravel! ❤️