refactor(datetime): Move behavior of DateTimeFormatter to standalone functions

[WWRS]

Improves test coverage.

The functions are almost entirely unchanged, with the exception of some renaming.

These functions should not be used outside tests, and DateTimeFormatter itself is an internal class.

TODO (I will look into these):

• Refactor errors to consistently provide useful feedback
• Add support for more format strings: Goal is parity with Intl.DateTimeFormat
• Add support for time zones (this will improve coverage in this file to almost 100%)

[codecov]

Codecov Report

Attention: Patch coverage is 95.11719% with 25 lines in your changes missing coverage. Please review.

Project coverage is 94.84%. Comparing base (c338b2f) to head (cbc909a).

Files with missing lines	Patch %	Lines
datetime/_date_time_formatter.ts	95.11%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6566      +/-   ##
==========================================
+ Coverage   94.75%   94.84%   +0.09%     
==========================================
  Files         584      584              
  Lines       46563    46601      +38     
  Branches     6541     6559      +18     
==========================================
+ Hits        44120    44199      +79     
+ Misses       2400     2358      -42     
- Partials       43       44       +1     

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