Added case() function

[ilongin]

Adding case() function.
Example:

  dc.mutate(
      res=func.case((C("num") > 0, "P"), (C("num") < 0, "N"), else_="Z"),
  )

[cloudflare-workers-and-pages]

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Latest commit:	958d2bb
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[amritghimire]

Looks like test cases are failing

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.33%. Comparing base (eb22c85) to head (958d2bb).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #770      +/-   ##
==========================================
- Coverage   87.33%   87.33%   -0.01%     
==========================================
  Files         116      116              
  Lines       11130    11145      +15     
  Branches     1528     1532       +4     
==========================================
+ Hits         9720     9733      +13     
- Misses       1031     1032       +1     
- Partials      379      380       +1     

Flag	Coverage Δ
datachain	87.26% <100.00%> (-0.01%)	⬇️

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