Add ClockType enum to core.time for issue# 13433.

This adds an enum for indicating which type of clock to use when it's
appropriate for a time function to have multiple options for the source
clock. Currently, it only supports "normal" and "coarse," but we may add
more in the future, since some systems do support others (e.g
clock_gettime has several other options depending on the system).

This is in support of https://github.com/D-Programming-Language/phobos/pull/2584.
Why does `MonoTimeImpl` not support it? Because I don't think that `CLOCK_SEC` a monotonic clock. I believe that it's a wall clock (though unfortunately, the documentation doesn't exactly go into a lot of detail on that - it can't be both though). So, I'd have to explicitly strip out the ticks beyond the second and wouldn't have any way to get the time faster other than whatever coarse is doing. So, I really don't see any point in supporting it for monotonic time.