Add standard timing functions

For e.g. `ease-in-out`

Example with JSS:

```
import {easeInOut} from 'css-functions'

const styles = {
container: {
transition: {
duration: 0.5,
timingFunction: easeInOUt,
}
}
}
```

No files or tests are named. Start by inspecting the repository's existing exported CSS functions and determine which standard timing functions are already represented; the work is done when the requested timing functions, including ease-in-out, are exposed consistently and the example can import and use them.

Scritto dal modello di indicizzazione a partire dal testo della issue.