Refactor `validateOptions()` utility (#5391)
Refactor `validateOptions()` utility (#5391)

- Improve types avoiding `any`
- Use `for...of` instead of `forEach` (`continue` is clearer than `return`)
- Use `flat()` instead of `[].concat()` idiom (avoiding type errors)
stylelint/stylelint,stylelint/stylelint,stylelint/stylelint,stylelint/stylelint