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.
I've never found a good page describing monotonic clocks vs realtime / wall clocks (that explanation is on `MonoTime` instead of `ClockType` anyway), and most of the specific clock types are pretty system-specific and not very well documented, which is why most of them just say things like `Uses $(D CLOCK_PROCESS_CPUTIME_ID)` and leaves it up to the programmer to sort that out. But no one should be using those unless they already know what they do anyway. The more complicated explanations are required in relation to what this code does with them when it's emulating them on systems that don't have them (which is what happens with `coarse`, `precise`, and `second`).