fix: output warning when built-in CSS support enabled

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

For better DX

Breaking Changes

No

Additional Info

part of webpack/webpack#17223, need to solve it for mini-css-extract-plugin and style-loader too

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (730f043) 96.81% compared to head (ff2b2a4) 96.84%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1520      +/-   ##
==========================================
+ Coverage   96.81%   96.84%   +0.02%     
==========================================
  Files          12       12              
  Lines        1131     1141      +10     
  Branches      411      418       +7     
==========================================
+ Hits         1095     1105      +10     
  Misses         27       27              
  Partials        9        9              

Impacted Files	Coverage Δ
src/index.js	100.00% <100.00%> (ø)

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