Format zero Durations as "0 seconds" instead of "0 microseconds"

[florish]

Summary of changes

While building a visual interface including a duration for a certain procedure, I noticed that calling the following:

0
|> Timex.Duration.from_seconds()
|> Timex.Format.Duration.Formatter.lformat(0, "en", :humanized)

...returns the string "0 microseconds". While technically 100% correct, from a "humanized" point of view, this feels a bit odd, as nobody would ever describe a zero-second duration as "zero microseconds".

This pull request is actually a change request, but as the core code change was minimal, opening a PR seemed like a more logical choice than opening an issue.

I'd like to receive feedback on (a) whether this is seen as a valuable functional change, and if so, (b) if you would like me to do the remaining work (fixing tests where needed, adding a CHANGELOG line) or that it is preferred to leave this to a core contributor.

Thanks for a great library!

Checklist

• New functions have typespecs, changed functions were updated
• Same for documentation, including moduledocs
• Tests were added or updated to cover changes
• Commits were squashed into a single coherent commit
• Notes added to CHANGELOG file which describe changes at a high-level