tstrsplit: add rev argument for reverse indexing (#6341)

[venom1204]

close #6341

this PR implements the rev argument in tstrsplit() to allow anchoring component extraction from the end of a string. This is particularly useful when strings have a varying number of delimiters and the user needs to extract the "last" or "second-to-last" elements.

hi @tdhock @joshhwuu can you have a look at this when you get time , thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (895067d) to head (c1f726a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7838   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          88       88           
  Lines       17266    17270    +4     
=======================================
+ Hits        17097    17101    +4     
  Misses        169      169           

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[github-actions]

• HEAD=issue6341 much slower for transform improved in #5493
  

Generated via commit c1f726a

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

Task	Duration
R setup and installing dependencies	2 minutes and 53 seconds
Installing different package versions	49 seconds
Running and plotting the test cases	6 minutes and 55 seconds