Add TypeScript declarations for configs

[FloEdelmann]

Extracted from https://github.com/vuejs/eslint-plugin-vue/issues/2166#issuecomment-2196820750 (by @pauliesnug):

As for full ESLint v9 support, which is related to the Flat Config milestone, we also need Type Declarations built-into the package so we can use type check configs.

That is not a breaking change and thus does not have to wait for v10.

Note that type declarations are also missing for other parts of the plugin, see #2124.