refactor(datetime): refactor DateTimeFormatter

[WWRS]

• Fix an issue where fractional seconds are not properly formatted
• Pull static functions out to allow for testing
• Add support for aa-aaaaa and MMM-MMMMM to get closer to parity with Intl (though the months aren't fully functional)
• Add value validation to "fractionalSecond"
• In formatToParts > "year"
  • Make it stricter: Reject years that are not exactly 2 or 4 digits
  • For 2-digit years, produce the 20XX value here instead of in partsToDate
• Rewrite partsToDate to collect all the parts and generate the final date in one step
  • Avoid doing any sorting or searching the array, which were used to handle short months: When generating a full date, short months are handled automatically
  • I think new Date(...) will be better for handling time zones
• Remove almost all as strings and handle errors caused by the cases where they're not strings
• Rewrite errors to match style guide
• Pull symbolToFormatPart out to make control flow in formatToFormatParts easier to understand
• Add a ton of tests, bringing coverage up to 100%
• Other minor refactors

TODO:

• Later: Add support for time zones
• Later: Add support for eras and months

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.45%. Comparing base (96116ce) to head (1ad8d30).
Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6509      +/-   ##
==========================================
- Coverage   95.50%   95.45%   -0.05%     
==========================================
  Files         575      576       +1     
  Lines       43279    43389     +110     
  Branches     6472     6542      +70     
==========================================
+ Hits        41335    41419      +84     
- Misses       1905     1929      +24     
- Partials       39       41       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[timreichen]

There are lots of different changes in this draft.
Would it be possible to split it up into multiple PRs?