make `frankenphp` directive optional

### Describe you feature request

all the defaults are sensible, now that workers can be defined in php_server blocks, we can skip it.

old config:
```Caddyfile
{
    frankenphp {
        worker /path/to/public/index.php
    }
}

http:// {
    root /path/to/public
    php
}
```

becomes
```Caddyfile
http:// {
    php {
        root /path/to/public
        worker index.php
    }
}
```