feat(isDate): add delimiter-less date formats

[xseman]

Add test cases for isDate validator to handle various delimiter-less date formats including YYYYMMDD, YYMMDD, MMDDYYYY and DDMMYYYY.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (c3dc37c) to head (09f0401).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2659   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2572      2601   +29     
  Branches       651       655    +4     
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+ Hits          2572      2601   +29     

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[xseman]

The MMDDYYYY and DDMMYYYY may be overkill, since they don't fall into the ISO 8601

[WikiRik]

Thanks for opening this PR! I think we should only accept these behind a non-default option, to not cause a breaking change

[xseman]

Thanks for the feedback, @WikiRik. What do you think about adding an opt-in for delimiter-less format support to avoid breaking changes?

const default_date_options = {
    format: 'YYYY/MM/DD',
    delimiters: ['/', '-'],
    strictMode: false,
    allowDelimiterless: false, // <----
};

// Existing behavior unchanged
validator.isDate('20020715'); // false

// Opt-in to new feature
validator.isDate('20020715', { 
    format: 'YYYYMMDD', 
    allowDelimiterless: true 
}); // true

[WikiRik]

@xseman yup, that's exactly what I had in mind!

[xseman]

Thanks for the quick feedback, last changes should do