devDeps: update 'prettier`

[juergba]

Description

• prevent peer dependency error by prettier:

mocha@9.1.3 D:\mocha-fork\mocha
├─┬ eslint-plugin-prettier@4.0.0
│ └── prettier@1.19.1 deduped invalid: ">=2.0.0" from node_modules/eslint-plugin-prettier
└── prettier@1.19.1 invalid: ">=2.0.0" from node_modules/eslint-plugin-prettier

npm ERR! code ELSPROBLEMS
npm ERR! invalid: prettier@1.19.1 D:\mocha-fork\mocha\node_modules\prettier

• many changes are forced by a new rule for function synthax: function() {} => function () {}
  There is no option to customize this setting (prettier is an opinionated code formatter).

[outsideris]

LGTM

[outsideris]

This PR includes a commit in #4775. (I think you already know it.)