Add %fin% operator for fast row-wise existence checks (#2279)

[venom1204]

closes #2279

adds the %fin% operator to perform optimized row-wise existence checks between data.tables. It supports multi-column matching using data.table join semantics and allows for the requested .(col1, col2) %fin% y syntax.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (813ed95) to head (4c053c0).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7880   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17309    17327   +18     
=======================================
+ Hits        17139    17157   +18     
  Misses        170      170           

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[github-actions]

• HEAD=issue2279 much slower for transform improved in #5493
  

Generated via commit 4c053c0

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

Task	Duration
R setup and installing dependencies	2 minutes and 58 seconds
Installing different package versions	23 seconds
Running and plotting the test cases	5 minutes and 37 seconds