feat: postcss@8

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

Use PostCSS 8

Breaking Changes

Yes

Additional Info

No

[codecov]

Codecov Report

Merging #1204 into master will decrease coverage by 0.21%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1204      +/-   ##
==========================================
- Coverage   99.40%   99.18%   -0.22%     
==========================================
  Files          11       22      +11     
  Lines         676     1356     +680     
  Branches      217      441     +224     
==========================================
+ Hits          672     1345     +673     
- Misses          4       11       +7     

Impacted Files	Coverage Δ
src/plugins/postcss-icss-parser.js	100.00% <100.00%> (ø)
src/plugins/postcss-import-parser.js	100.00% <100.00%> (ø)
src/plugins/postcss-url-parser.js	100.00% <100.00%> (ø)
src/utils.js	98.08% <0.00%> (-0.64%)	⬇️
D:/a/css-loader/css-loader/src/cjs.js	100.00% <0.00%> (ø)
...ader/css-loader/src/plugins/postcss-icss-parser.js	100.00% <0.00%> (ø)
D:/a/css-loader/css-loader/src/CssSyntaxError.js	100.00% <0.00%> (ø)
D:/a/css-loader/css-loader/src/Warning.js	100.00% <0.00%> (ø)
D:/a/css-loader/css-loader/src/runtime/getUrl.js	100.00% <0.00%> (ø)
D:/a/css-loader/css-loader/src/utils.js	98.40% <0.00%> (ø)
... and 5 more

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