feat: add one-letter-css plugin

[denisx]

This PR contains a:

• [ ] bugfix
• [x] new feature
• [ ] code refactor
• [x] test update
• [ ] typo fix
• [x] metadata update

resolve issue #1028

For efficient gzip/br compression, plugin combine css hash via one symbol name, as a classname position at file, with filepath hash:base64:8, to have strong sequences.

[codecov[bot]]

Codecov Report

Base: 96.81% // Head: 98.83% // Increases project coverage by +2.01% :tada:

Coverage data is based on head (1f513e9) compared to base (7c5cdcc). Patch coverage: 97.16% of modified lines in pull request are covered.

:exclamation: Current head 1f513e9 differs from pull request most recent head b06b840. Consider uploading reports for the commit b06b840 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1181      +/-   ##
==========================================
+ Coverage   96.81%   98.83%   +2.01%     
==========================================
  Files          12       12              
  Lines        1131      773     -358     
  Branches      411      236     -175     
==========================================
- Hits         1095      764     -331     
+ Misses         27        9      -18     
+ Partials        9        0       -9     

Impacted Files	Coverage Δ
src/plugins/hash-len-suggest.js	90.00% <90.00%> (ø)
src/plugins/one-letter-css.js	100.00% <100.00%> (ø)
src/cjs.js	100.00% <0.00%> (ø)
src/index.js	100.00% <0.00%> (ø)
src/Warning.js	100.00% <0.00%> (ø)
src/runtime/api.js	100.00% <0.00%> (ø)
src/CssSyntaxError.js	100.00% <0.00%> (ø)
src/runtime/getUrl.js	100.00% <0.00%> (ø)
src/plugins/postcss-icss-parser.js	100.00% <0.00%> (ø)
src/plugins/postcss-import-parser.js	100.00% <0.00%> (ø)
... and 4 more

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[linux-foundation-easycla[bot]]

• :x: - login: @denisx / name: denisx . The commit (bbe543ae0b22ee95117785a4f3a6676d55fb5093, c7421642525ab5b5366886c5a0de69dac888d0c0, d5c9c1d0fd43aee91dac83bff65e03ed3ac93c1a, c2938f863480fb26e98f31631df502dcb8874266, 1f513e922a702f43934111ef04e60703bf812580, 81859519f23026115e67d2afe72ba56e63718a5a, b06b840ac7cad421cba34de50de88ba804fdcad4) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[TKDev7]

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