Increase number of allowed digits when parsing ISO duration

[thomas-darling]

Fixes #880.

The regex for parsing an ISO duration string restricts the number of fractional digits to 9, with the exception of fractional seconds, which is restricted to 20. However, the toISO method outputs up to 16 fractional digits, which means the output of toISO may not be parsable by fromISO.

This fix increases the number of allowed digits to 20 for all units.

[linux-foundation-easycla]

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• ✅ login: thomas-darling / name: Thomas Darling (01ad50b)

[icambron]

Thanks!