Add getter/setter functions for `DigitsFormat`/`Accuracy`

[LukeSaward]

Currently, it seems impossible to access the DigitsFormat and Accuracy in use by an instance of the Time and Fraction structs respectively (contained within livesplit_core::timing::formatter::timer, since there are no getter functions for either.

I originally planned to publicize the digits_format and accuracy fields of Time and Fraction structs respectively, but decided getter/setter functions would be a better approach.