[STYLELINT] Fix StyleLint report - CSS code style improvements

Tommy Tholasrequested to merge
issue/dify-3539940:3539940-fix-stylelint-report into 1.0.x
  • Add .stylelintrc.json configuration with stylelint-config-standard
  • Add .stylelintignore to exclude third-party CSS files
  • Fix color-hex-length: #ffffff → #fff
  • Fix length-zero-no-unit: 0px → 0
  • Configure selector-class-pattern to support BEM notation (__)

Closes #3539940

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