Fix YAML date parsing

[kytta]

This PR makes a few changes to how date is parsed in YAML preambles. Fixes #2538.

Description

New regex

Initially, I planned to address #2538 by adding the fraction separator (.) to the optional group so that it also becomes optional. However, I have made a few more changes:

• named groups: instead of indexed access, one can access date components via their name
• Parsing is now a bit off-spec
  • when one reads the timestamp working draft, one can see that they use two different parts for dates with and without time and tz
  • the former (just date) does not allow for short months and days (i.e. without leading zero), and is thus compliant with ISO 8601
  • the latter (date, time, and optional offset) allow months, days, and hours without leading zeroes. This is non-compliant, but user-friendly
  • I find it weird that both variants are present and have made the choice to allow months and days without leading zeroes in both 'short' and 'long' forms
• as a side effect, there is no group duplication any more, and the regex is now shorter
• I also make use of non-capturing groups to split the regex more cleanly

You can explore the regex on regex101: h5wyK4; and RegExr: 824pi

Fixes offset parsing

As far as I can understand from code, the parsing of the offset was done incorrectly:

• to convert to nanosecond, seconds were multiplied by 100 million (instead of 1 billion)
• instead of a floating point number, an integer was multiplied
• the integer was parsed from the decimal part without the dot

I have revised the code and arrived at this solution:

• the regex allows up to 9 decimal points in the fraction part
• the fraction part get parsed as f64 number. It is between 0 and 1 (exclusive)
• this floating point number represents a fraction of a second
• this floating point number gets multiplied by one billion (nanoseconds in a second) and applied to the offset

New tests

I have re-organized some tests and have added some more tests. I've taken the new test cases from the TOML test suite, but have only taken those that make sense with the YAML spec

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Code changes

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If the change is a new feature or adding to/changing an existing one:

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[kytta]

Ooops; I've hoped that [skip ci] would work for Azure 😅

[kytta]

Don't mind me, it's easier for me to track what I need to do when I return to this

[kytta]

@Keats I've fixed the fraction separator being required and have further optimized the regex. I've added some tests (test cases courtesy of TOML); if you don't mind them overlapping, I think it's a good test suite for this functionality 😄

[Keats]

Neat, thanks