[9.x] Add skip cancelled middleware

[morrislaptop]

This PR adds a small Job middleware which will skip Batchable jobs if the associated batch is cancelled.

This basically removes the need for an initial if statement from the handle method:

/**
 * Execute the job.
 *
 * @return void
 */
public function handle()
{
    if ($this->batch()->cancelled()) {
        return;
    }
 
    // Continue processing...
}

To this:

public function middleware()
{
    return [new SkipCancelled];
}

Which aligns with the purpose of middleware from the docs

Job middleware allow you to wrap custom logic around the execution of queued jobs, reducing boilerplate in the jobs themselves.

[driesvints]

Thanks! @morrislaptop can you also send a PR to the docs?

[morrislaptop]

@driesvints laravel/docs#8495