Allow Font size properties to accept strings?

### Feature Proposal

The `size` property of [Fonts](https://www.chartjs.org/docs/latest/general/fonts.html) can be either a number OR a string, to allow for relative or other units like `2rem`.

### Feature Use Case

Our app follows a design system where fonts are sized by `rem` values, and using a relative value like `rem` or `em` is the recommended best practice for responsive web design.

## Possible Implementation

I hope it would be not difficult to implement, but if there's some hidden reason this would be very challenging, I understand that too.