Add `DateTimePeriod` parser

Would allow us to parse Strings like "P2Y4M-1D" into: two years, four months, minus one day; creating a `kotlinx.datetime.DateTimePeriod`.

Blocked by https://github.com/Kotlin/kotlinx-datetime/issues/615

Start by reading the linked kotlinx-datetime issue 615, since this request is explicitly blocked by it. Then inspect the existing date and period parsing entry points in the repository, if available, and use the example "P2Y4M-1D" as the acceptance case; done means it produces a kotlinx.datetime.DateTimePeriod with the stated components.

Written by the indexing model from the issue text.