fix(isISO8601): reject week-zero dates (W00 is never a valid ISO 8601 week)

[JSap0914]

Bug

isISO8601 accepts W00 week dates as valid ISO 8601, but ISO 8601 week dates always start at W01 — week zero does not exist.

isISO8601('2024-W00')   // → true  ← false-positive
isISO8601('2024-W00-1') // → true  ← false-positive

Reference: ISO 8601 § Week date — "The first week of the year contains the first Thursday of the year" and is numbered W01; W00 is never assigned.

Fix

In both iso8601 and iso8601StrictSeparator regexes, change the week-number alternation from:

W([0-4]\d|5[0-3])   ← matches W00–W53

to:

W(0[1-9]|[1-4]\d|5[0-3])   ← matches W01–W53

Verification

$ npm test
318 passing (416ms)

All 318 existing tests pass. Four new invalid-case tests added: '2009-W00', '2009-W00-1', '2024-W00', '2020-W00-7'.

Checklist

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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (7fdc788) to head (c09116c).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2774   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          114       114           
  Lines         2587      2587           
  Branches       656       656           
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  Hits          2587      2587           

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