feat(deprecation): deprecate the extend function

[misteroneill]

Description

Related to #7943, see that for details.

Specific Changes proposed

• Deprecates the extend function

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #7944 (79ca933) into main (5bcf4d1) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #7944      +/-   ##
==========================================
+ Coverage   80.94%   80.95%   +0.01%     
==========================================
  Files         116      116              
  Lines        7467     7471       +4     
  Branches     1816     1817       +1     
==========================================
+ Hits         6044     6048       +4     
  Misses       1423     1423              

Impacted Files	Coverage Δ
src/js/extend.js	85.71% <100.00%> (+3.36%)	⬆️

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