Should use a separate `format` script instead of relying on `eslint --fix`

[haoqunjiang]

See discussions at typescript-eslint/typescript-eslint#4907 and Prettier's official guidance https://prettier.io/docs/en/integrating-with-linters.html

This means we should stop using @vue/eslint-config-prettier (which turns on the prettier/prettier rule), but depending on eslint-config-prettier directly.

Changes need to be landed in https://github.com/vuejs/create-eslint-config

---

Things might get trickier if we take airbnb & standard style rules into consideration, though.