Incorporate `inf-php`

Takayuki Sato's [`inf-php`](https://github.com/taksatou/inf-php) package provides a useful way to run an interactive PHP session. It is a much better version of the existing `php-send-region` command. We should remove that command and defer to `inf-php` instead. Futhermore, we also need to make sure that no key-bindings in `inf-php` conflict with bindings in PHP Mode.