feat(flatten): add flatten

[ssi02014]

close: #137

Implement the same function as lodash's flattenDepth under the name flatten(arr, depth).

It works the same as Array.prototype.flat provided by default in JavaScript and returns the same type. However, its performance is superior.

benchmark

[vercel]

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (3c8d105) to head (ecba55b).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #147   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           68        69    +1     
  Lines          384       395   +11     
  Branches        49        52    +3     
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+ Hits           384       395   +11     

[raon0211]

Thanks for your great contribution!