Similar to #7475, but unlike timeout and sleep (#7669), we do not support second/minute/hour/day specifier.

GNU coreutils supports hex formatting for the parameter -s, a.k.a. --sleep-interval=N:

tail -s 0x1 -f
tail -s 0x1.1 -f
tail -s 0x1.1p-1 -f

Not a huge deal, probably a pretty rare edge case that I doubt anybody cares about, but it seems worth it to use generic float parsing functions in uucore/parser.

Together with #7669, that would allow to remove dependency on fundu.