Use php:X.Y instead of PHP.X.Y

Currently, the target parameter expects `PHP.7.4`, which may be a bit awkward. Extend the compiler to support using a `-` here, e.g:

```bash
# Target PHP 7.4 (default target is current PHP version)
$ xp compile -t php:7.4 HelloWorld.php HelloWorld.class.php
```

This notation is more likely to be known from people, e.g. because it's how Docker separates its images and tags.