Add ISO-8601 style Duration parsing

[sffc]

It would be convenient for users and for writing unit tests if we could parse ISO-8601 duration strings into DateDuration.

Examples:

• P1D => { days: 1 }
• P2M1D => { months: 2, days: 1 }
• P77Y => { years: 77 }

[KekmaTime]

@sffc i can work on this issue

[sffc]

Thanks! You can learn more about the duration syntax here:

https://en.wikipedia.org/wiki/ISO_8601#Durations

https://tc39.es/proposal-temporal/#prod-TemporalDurationString

Since DateDuration supports only years, months, weeks, and days, we should parse only the date fields from duration strings, and reject duration strings that have time portions after the T. So, for example, we should parse P5M but not PT5M.

This should be exported as a function DateDuration::try_from_str and DateDuration::try_from_utf8 with a corresponding FromStr impl. You can see lots of other examples of signatures of parsing functions throughout ICU4X.