Implement initcap/concat functions

[acquamarin]

This PR implements the inicap and concat functions as requested by the user.

[codecov]

Codecov Report

Attention: Patch coverage is 97.87234% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 92.08%. Comparing base (c747899) to head (c380ed8).
Report is 3 commits behind head on master.

Files	Patch %	Lines
src/function/vector_string_functions.cpp	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3161      +/-   ##
==========================================
- Coverage   92.08%   92.08%   -0.01%     
==========================================
  Files        1168     1168              
  Lines       44087    44104      +17     
==========================================
+ Hits        40598    40611      +13     
- Misses       3489     3493       +4     

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