feat: add xyFromInterleaved and xyToInterleaved

[lpatiny]

Summary

• Add xyFromInterleaved to convert a flat [x, y, x, y, ...] array to a DataXY object
• Add xyToInterleaved to convert a DataXY object back to a flat interleaved array
• Both functions tested with basic cases, edge cases (empty, Float64Array), and a round-trip test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (904d97c) to head (210820b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
+ Coverage   97.14%   97.15%   +0.01%     
==========================================
  Files         204      206       +2     
  Lines        4100     4116      +16     
  Branches     1031     1031              
==========================================
+ Hits         3983     3999      +16     
  Misses        113      113              
  Partials        4        4              

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[lpatiny]

@targos Why it waits for node 20 ? Can you fix this ?