Simplify date parser

[CrazyboyQCD]

• Avoid allocation if string is ascii
• Return early if string from utf16 encoded JsString is not ascii
• Replace Char<'_> with slice::Iter<'_, u8>
• Add next_n_digits helper method to save calls of next_digit
• Merge duplicate codes in matches

[codecov]

Codecov Report

Attention: Patch coverage is 72.09302% with 24 lines in your changes missing coverage. Please review.

Project coverage is 53.67%. Comparing base (6ddc2b4) to head (51974b7).
Report is 364 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/builtins/date/utils.rs	72.09%	24 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4143      +/-   ##
==========================================
+ Coverage   47.24%   53.67%   +6.42%     
==========================================
  Files         476      487      +11     
  Lines       46892    51811    +4919     
==========================================
+ Hits        22154    27808    +5654     
+ Misses      24738    24003     -735     

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[nekevss]

Looks good to me at a high level. I like the move from char to u8 ascii. I have a suggestion.