Units unary operators are not constexpr

[akmin04]

Describe the bug
Unary operators on units are not constexpr. Fixed in nholthaus/units#226, but the change not reflected in wpilib's units files (I'm not sure how/when wpilib's version of the units library is updated)

constexpr meter_t x = 1_m; works
constexpr meter_t x = -1_m; doesn't

Desktop (please complete the following information):

• WPILib Version: 2022.4.1

[calcmogul]

We've permanently forked from them at this point. Having one header with every unit in it had measurably worse compilation time and compiler memory usage compared to separate headers for each unit and a base header for common macro definitions.

When we get C++20, we plan to look into migrating to https://github.com/mpusz/units/.