Add parseISODuration function

[ckcherry23]

This is a continuation of PR #3151 by @matinzd.

Mainly I have:

• Fixed the docs formatting
• Added more edge cases in the tests
• Made it invalid to use durations with just P or nothing after T as per the Wikipedia page
• However, we still accept decimal values for any unit (eg. P0.5YT1M), even though the standard accepts decimal values only for the last unit specified (eg. P0.5Y)

[kossnocorp]

I don't like the idea of assigning undefined values to 0. I know that formatISODuration assigns zero values, but these two functions together lose information. I recently played with Temporal's duration, and there you can format(parse('P1Y2M')) === 'P1Y2M', which I think is a very good approach. While we can't change formatISODuration in v3, we can move in that direction.

WDYT?

[ckcherry23]

I agree! I've updated the implementation to preserve undefined values instead of assigning them to 0.