Extend add operation for list,string

[acquamarin]

Extend add operation to support
STRING + STRING -> STRING
VARLIST + VARLIST -> VARLIST

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0763648) 90.80% compared to head (f1b6f13) 90.82%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2444      +/-   ##
==========================================
+ Coverage   90.80%   90.82%   +0.02%     
==========================================
  Files        1023     1023              
  Lines       37023    37032       +9     
==========================================
+ Hits        33617    33635      +18     
+ Misses       3406     3397       -9     

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