Add Sedate IXDTF (extended ISO) string parsing function

[sffc]

We should add a function that parses ISO-8601 strings. We should follow the Temporal grammar:

https://tc39.es/proposal-temporal/#sec-temporal-iso8601grammar

i.e., we should implement a string parser conforming to IETF Sedate that parses a string into its components (date, time, time zone, calendar), and use this parser in APIs and anywhere else we parse date strings. Think about where to put this code in the crate structure.

We should start with the DateTime production, and work our way up towards CalendarDateTime and TemporalZonedDateTimeString.

Examples of strings that are accepted as part of the DateTime grammar:

• 2022-06-27T16:31:00.000
• 2022-06-27t16:31:00.000
• 2022-06-27 16:31:00.000
• 20220627 16:31:00.000
• 20220627 163100.000
• 20220627 163100
• 20220627 1631

[sffc]

Note: currently we have https://unicode-org.github.io/icu4x-docs/doc/icu_datetime/mock/fn.parse_gregorian_from_str.html

We should start with a design doc for this. We likely want to end up with a parser type named something like IetfDateTimeStringParser that is able to support the whole grammar.

[sffc]

Question: do we want to have specific, stricter parsers for each individual type, as well as the fully-featured one conforming to IETF Sedate?