useFlatConfig enabled by default?

[aladdin-add]

The flat config in ESLint has reached stability and will be enabled by default in ESLint v9. To activate it in vscode-eslint, use "eslint.experimental.useFlatConfig": true(the default is false). It seems like an opportune moment to enable it by default. The API shouldUseFlatConfig() can be employed for checking:

const {shouldUseFlatConfig} = require("eslint/use-at-your-own-risk");

shouldUseFlatConfig(); // => ...

refs: https://eslint.org/blog/2023/10/flat-config-rollout-plans/

[aladdin-add]

/dupl #1644