feat(countBy): Add countBy function

[l2hyunwoo]

close #88

I added countBy function that count the occurrences of each item in an array based on a transformation function.

[vercel]

@l2hyunwoo is attempting to deploy a commit to the Toss Team on 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 (e7472e0) to head (e8c1e25).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #117   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           58        60    +2     
  Lines          314       325   +11     
  Branches        37        39    +2     
=========================================
+ Hits           314       325   +11     

[codspeed-hq]

CodSpeed Performance Report

Merging #117 will degrade performances by 88.97%

_{Comparing l2hyunwoo:feature/#88 (44f33b4) with l2hyunwoo:feature/#88 (e8c1e25)}

Summary

⚡ 3 improvements
❌ 7 regressions
✅ 98 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	l2hyunwoo:feature/#88	l2hyunwoo:feature/#88	Change
⚡	es-toolkit/countBy	70.2 µs	55.5 µs	+26.37%
❌	lodash/difference	56.7 µs	337.1 µs	-83.17%
❌	lodash/intersectionWith	92 µs	263.2 µs	-65.05%
❌	lodash/isNil	35.3 µs	319.7 µs	-88.97%
❌	es-toolkit/range	39.2 µs	48.3 µs	-18.84%
❌	lodash/range	33.4 µs	44.2 µs	-24.59%
⚡	lodash/unionBy	232.7 µs	75.4 µs	×3.1
❌	lodash/unionWith	84.8 µs	239.8 µs	-64.64%
⚡	es-toolkit/uniqWith	44.4 µs	38.4 µs	+15.52%
❌	lodash/xor	255.6 µs	397.5 µs	-35.68%

[raon0211]

Thanks for your great contribution!