support for generation @apply directives

sometimes you need to add custom css classes because some libraries need it. It should be easy to allow generation of:
```
.phx-modal {
  @apply opacity-100 fixed left-0 top-0 w-full h-full overflow-auto;
}
```
from 
```
.phx-modal {
  opacity: 1!important;
  position: fixed;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
}
```