fix stylelint errors and upgrade @apostrophecms/stylelint-no-mixed-decls

[ETLaurent]

Summary

Relies on apostrophecms/stylelint-no-mixed-decls#2

• fix current sass error done in Pro 8178 add prepend append main #5068
• use the new @apostrophecms/stylelint-no-mixed-decls plugin option to define external mixins known to contain nested rules

By adding apos-transition to the contain-nested option array, stylelint will complain, like sass.

{
  "extends": "stylelint-config-apostrophe",
  "rules": {
    "@apostrophecms/stylelint-no-mixed-decls": [
      true,
      {
        "contain-nested": [ "apos-transition" ]
      }
    ]
  }
}

[ETLaurent]

config moved to apostrophecms/stylelint-config-apostrophe#32