feat: add lowercase

[yeonjuan]

No description provided.

[codecov]

Codecov Report

Merging #158 (c226d51) into main (32ebe14) will increase coverage by 0.05%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   98.14%   98.19%   +0.05%     
==========================================
  Files          44       45       +1     
  Lines         968      995      +27     
  Branches      251      257       +6     
==========================================
+ Hits          950      977      +27     
  Misses         18       18              

Flag	Coverage Δ
unittest	98.19% <100.00%> (+0.05%)	⬆️

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Files	Coverage Δ
packages/eslint-plugin/lib/rules/index.js	100.00% <100.00%> (ø)
packages/eslint-plugin/lib/rules/lowercase.js	100.00% <100.00%> (ø)