refactor(datetime): Split DateTimeFormatter functions out

[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 94.72656% with 27 lines in your changes missing coverage. Please review.

Project coverage is 95.31%. Comparing base (b5a5fe4) to head (471a374).

Files with missing lines	Patch %	Lines
datetime/_date_time_formatter.ts	94.72%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6566      +/-   ##
==========================================
+ Coverage   95.21%   95.31%   +0.09%     
==========================================
  Files         577      577              
  Lines       43381    43419      +38     
  Branches     6487     6505      +18     
==========================================
+ Hits        41307    41385      +78     
+ Misses       2034     1994      -40     
  Partials       40       40              

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