added isoyear funtion

[venom1204]

Closes #7154

This PR adds isoyear() as a complement to isoweek(), providing the ISO 8601 year for given dates.

Hi @MichaelChirico, could you please review this when you have a moment?
Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.77%. Comparing base (428ba9a) to head (e6d6512).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7197   +/-   ##
=======================================
  Coverage   98.77%   98.77%           
=======================================
  Files          81       81           
  Lines       15215    15216    +1     
=======================================
+ Hits        15029    15030    +1     
  Misses        186      186           

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[github-actions]

• HEAD=issue7154 slower P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit e6d6512

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 48 seconds
Installing different package versions	20 seconds
Running and plotting the test cases	2 minutes and 37 seconds

[ben-schwen]

LGTM, TY!