feat: support for multi character ISO time format notation

[D-Maxwell]

hello there ;

I would quite appreciate the ability to make use of other variables aside from the standard single character ones that eza currently expects through its time-style argument.

For instance, DDD is the ISO standard for days since the start of the year, and I wish it were usable in eza for personal ricing. Besides, multiple characters are meant to indicate how many digits each segment of a date should be "reserved" ; that is, how many leading zeroes should be accounted for.