Polyfill: Update Meiji era start date

[ptomato]

Update internal era data to change hasYearZero to startingYear, so that we can accommodate the starting year of 6. Update test262 commit to pull in tests for this.

Normative change, approved in 2026-01 TC39: tc39/proposal-intl-era-monthcode#102

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.94%. Comparing base (fe12f07) to head (0f8268e).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3263      +/-   ##
==========================================
+ Coverage   97.89%   97.94%   +0.04%     
==========================================
  Files          22       22              
  Lines       10405    10401       -4     
  Branches     1819     1814       -5     
==========================================
+ Hits        10186    10187       +1     
+ Misses        200      196       -4     
+ Partials       19       18       -1     

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[Ms2ger]

Thanks!