Support flat config in eslint-config-vuepress

[Mister-Hope]

We can use flat config in v8.57.0. This benefits the speed of eslint and make it easier to override things.

We can have plugins built-in in eslint-config-vuepress also, such as eslint-plugin-vitest and eslint-plugin-import. Check https://github.com/vuepress-theme-hope/vuepress-theme-hope/blob/main/eslint.config.js as an example.

https://github.com/Mister-Hope/eslint-config-hope/blob/main/src/index.ts

[Mister-Hope]

@meteorlxy It's time to relook into this now, as typescript-eslint support eslint v9, there is currently no blocking things.

I release eslint-config-mister-hope with eslint v9 and it works perfectly in all my projects.