Implements the --resolution flag for date. On unix platforms this uses the posix function clock_getres. I could not find an equivalent syscall on windows, however every windows operating system since at least Windows 2000 has used a resolution of 100ns, so I decided to hardcode that as the returned resolution.

I treated the --resolution flag as a date source like it is in GNU date, this allows other formatting flags to apply to the output of the --resolution flag. I also tried to make sure the implementation would interact well with a future implementation of the --debug flag.

As a bit of a drive by I also add conflicts_with annotations to the four date source options, this is to make the GNU date behaviour of failing when multiple date sources are specified. However, I'm not sure if I've taken the best approach for this, since the error message is different to GNU make and I'm not sure whether or not this is an issue.

date -d "2025-01-01" --resolution

gnu date:
date: the options to specify dates for printing are mutually exclusive
Try 'date --help' for more information.

this pr:
error: the argument '--date <STRING>' cannot be used with '--resolution'

For more information, try '--help'.

fixes #6143