Support for `datetime` based axis

It would be nice to be able to plot data, where one of the axes is a date or datetime. Currently, when trying to do this it will cause a compiler error

```
expected integer, float, length, angle, ratio, fraction, or decimal, found duration
```

[in axis.typ](https://github.com/cetz-package/cetz-plot/blob/8002618c9c0c9b3809d99d2dc66b5e2ca76f6b85/src/axes.typ#L453)