feat(postcss): support postcss v8 with explict postcss installation

[clarkdo]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

As we can't fully migrated to postcss 8 until postcss-preset-env supports v8

This pr is for supporting postcss config in nuxt if user installs postcss packages explicitly, required packages:

NOTE: postcss-preset-env is disabled in v8 config as it doesn't support postcss v8 for now.

yarn add --dev css-loader@^5.0.0 postcss@^8.1.10 postcss-import@^13.0.0 postcss-loader@^4.1.0 postcss-url@^10.1.1
# or
npm i --save-dev css-loader@^5.0.0 postcss@^8.1.10 postcss-import@^13.0.0 postcss-loader@^4.1.0 postcss-url@^10.1.1

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.

[clarkdo]

Most code is duplicate of postcss.js, this is more handy when we fully migrate to v8: just remove postcss.js and rename postcss-v8.js

[codecov-io]

Codecov Report

Merging #8546 (4565e5d) into dev (b2e38f5) will decrease coverage by 1.46%.
The diff coverage is 6.31%.

@@            Coverage Diff             @@
##              dev    #8546      +/-   ##
==========================================
- Coverage   67.46%   66.00%   -1.47%     
==========================================
  Files          91       92       +1     
  Lines        3916     4009      +93     
  Branches     1082     1104      +22     
==========================================
+ Hits         2642     2646       +4     
- Misses       1025     1095      +70     
- Partials      249      268      +19     

Flag	Coverage Δ
unittests	66.00% <6.31%> (-1.47%)	⬇️

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Impacted Files	Coverage Δ
packages/webpack/src/utils/postcss-v8.js	1.12% <1.12%> (ø)
packages/webpack/src/utils/style-loader.js	74.57% <83.33%> (+0.03%)	⬆️

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